Plated steel sheet and manufacturing method therefor

Abstract

The present invention provides a plated steel sheet comprising: a base steel sheet; and a plating layer on the base steel sheet, the base steel sheet comprising, by wt%, 0.1-0.4% of carbon (C), silicon (Si) in an amount greater than 0% and less than or equal to 2.0%, 1.5-4.0% of manganese (Mn), aluminum (Al) in an amount greater than 0% and less than or equal to 2.0%, phosphorus (P) in an amount greater than 0% and less than or equal to 0.02%, sulfur (S) in an amount greater than 0% and less than or equal to 0.005%, nitrogen (N) in an amount greater than 0% and less than or equal to 0.006%, boron (B) in an amount greater than 0% and less than or equal to 0.003% and the balance of iron (Fe) and other inevitable impurities, wherein the sum of the amount of silicon (Si) and the amount of aluminum (Al) in the base steel sheet is 2.0% or less, and the ratio of the amount of silicon (Si) to the amount of aluminum (Al) is less than 4.0.

Description

Galvanized steel sheet and its manufacturing method

[0001] The present invention relates to a steel plate and a method for manufacturing the same, and more particularly, to a plated steel plate and a method for manufacturing the same.

[0002] Automotive steel plates have been developed with a focus on increasing strength to ensure user safety and reduce vehicle weight, and securing elongation for easy processing. The most common ultra-high-strength steels currently in use include dual-phase steel (DP steel), which secures elongation through two phases of ferrite and martensite, and transformation-induced plasticity steel (TRIP steel), which secures strength and elongation through the phase transformation of retained austenite in the final structure during plastic deformation by controlling the phase transformation of austenite during heat treatment. However, development based on DP steel, which cannot overcome the limits of the rule of mixture (ROM), and TRIP steel, which is composed of a ferrite matrix with some bainite and tempered martensite and has relatively low strength, has reached its limit. Therefore, there is a need to develop next-generation ultra-high-strength automotive steel sheets that can improve the microstructure of transformation-induced plasticity steel to achieve the ultra-high strength and high formability demanded by customers. Prior art literature includes Korean Patent Publication No. 20200075949A.

[0003] The problem to be solved by the present invention is to provide an ultra-high-strength plated steel sheet having excellent formability and weldability while overcoming the limitations of the mechanical properties of existing DP steel or TRIP steel, and a method for manufacturing the same.

[0004] However, these tasks are exemplary and the technical idea of ​​the present invention is not limited thereto.

[0005] According to one aspect of the present invention, a plated steel sheet is provided. The plated steel sheet comprises a base steel sheet containing, in wt%, carbon (C): 0.1 to 0.4%, silicon (Si): more than 0% and 2.0% or less, manganese (Mn): 1.5 to 4.0%, aluminum (Al): more than 0% and 2.0% or less, phosphorus (P): more than 0% and 0.02% or less, sulfur (S): more than 0% and 0.005% or less, nitrogen (N): more than 0% and 0.006% or less, boron (B): more than 0% and 0.003% or less, and the remainder including iron (Fe) and other unavoidable impurities; and a plating layer on the base steel sheet; wherein the sum of the content of silicon (Si) and the content of aluminum (Al) in the base steel sheet is 2.0% or less, and the ratio of the content of silicon (Si) to the content of aluminum (Al) is less than 4.0.

[0006] In the above-mentioned plated steel sheet, the final microstructure of the base steel sheet can satisfy the following equation 1.

[0007] (Formula 1)

[0008] (X Pearlite + X Fresh Martensite ) / (100 - X Ferrite - X Tempered Martensite - X Bainite ) ≤ 0.30

[0009] (above X Pearlite is the value of the area fraction of pearlite (unit: %), and the above X Fresh Martensite is the value of the area fraction of fresh martensite (unit: %), and the above X Ferrite is the value of the area fraction of ferrite (unit: %), and X Tempered Martensite is the value of the area fraction of tempered martensite (unit: %), and the above X Bainite is the value of the area fraction of bainite (unit: %)

[0010] A method for manufacturing a plated steel sheet according to another aspect of the present invention is provided. The method for manufacturing the plated steel sheet comprises the steps of hot-rolling a steel material to provide a hot-rolled steel sheet, the steel material containing, in wt%, carbon (C): 0.1 to 0.4%, silicon (Si): more than 0% and 2.0% or less, manganese (Mn): 1.5 to 4.0%, aluminum (Al): more than 0% and 2.0% or less, phosphorus (P): more than 0% and 0.02% or less, sulfur (S): more than 0% and 0.005% or less, nitrogen (N): more than 0% and 0.006% or less, boron (B): more than 0% and 0.003% or less, and the remainder including iron (Fe) and other inevitable impurities, wherein the sum of the content of silicon (Si) and the content of aluminum (Al) is 2.0% or less, and the ratio of the content of silicon (Si) and the content of aluminum (Al) is less than 4.0; A method for producing a cold-rolled steel sheet, the method comprising: a step of cold-rolling the hot-rolled steel sheet to provide a cold-rolled steel sheet; a step of annealing the cold-rolled steel sheet; a step of slowly cooling the annealed steel sheet at a first cooling rate; a step of rapidly cooling the slowly cooled steel sheet at a second cooling rate greater than the first cooling rate to a temperature lower than a martensite transformation initiation temperature (Ms); a step of reheating the rapidly cooled steel sheet to a temperature range higher than the martensite transformation initiation temperature (Ms) and lower than the bainite transformation initiation temperature (Bs); a step of performing a plating treatment on the reheated steel sheet to form a plating layer on the base steel sheet; and a step of alloying the base steel sheet and the plating layer to a temperature range of 480 to 560°C and then cooling them to room temperature.

[0011] In the above method for manufacturing the plated steel sheet, the hot rolling can be performed under the conditions of a reheating temperature of 1150 to 1250°C, a finishing rolling temperature of 800 to 950°C, and a coiling temperature of 400 to 650°C.

[0012] In the above method for manufacturing a plated steel sheet, the first cooling rate may be lower than 20°C / s, the second cooling rate may be higher than 20°C / s, the slow cooling end temperature of the slow cooling step may be 650 to 750°C, and the rapid cooling end temperature of the rapid cooling step may be 200 to 300°C.

[0013] In the above method for manufacturing the plated steel sheet, the alloying heat treatment step includes a step of maintaining the temperature range of 480 to 560°C for 15 to 45 seconds and then cooling to room temperature at a third cooling rate, wherein the third cooling rate may be greater than the first cooling rate and less than the second cooling rate.

[0014] In the method for manufacturing the above-mentioned plated steel sheet, the final microstructure of the base steel sheet implemented after performing the alloying heat treatment step can satisfy the following mathematical formula 1.

[0015] (Formula 1)

[0016] (X Pearlite + X Fresh Martensite ) / (100 - X Ferrite - X Tempered Martensite - X Bainite ) ≤ 0.30

[0017] (above X Pearlite is the value of the area fraction of pearlite (unit: %), and the above X Fresh Martensite is the value of the area fraction of fresh martensite (unit: %), and the above X Ferrite is the value of the area fraction of ferrite (unit: %), and X Tempered Martensite is the value of the area fraction of tempered martensite (unit: %), and the above X Bainite is the value of the area fraction of bainite (unit: %)

[0018] According to the present invention, it is possible to realize an ultra-high-strength plated steel sheet and a manufacturing method thereof that have excellent formability and weldability while overcoming the limitations of the mechanical properties of existing DP steel or TRIP steel.

[0019] The effects of the present invention described above are illustrative, and the scope of the present invention is not limited by these effects.

[0020] Figure 1 is a graph showing the results of a glib tensile test on the plated steel sheet and the base steel sheet of the present invention.

[0021] FIG. 2 is a drawing illustrating the results of simulating the movement of the interface between the plating layer and the base steel sheet and the aluminum diffusion pattern immediately after spot welding in a plated steel sheet according to one embodiment of the present invention.

[0022] FIG. 3 is a drawing illustrating a change in stability of an interface due to aluminum diffusion in a plated steel sheet according to one embodiment of the present invention.

[0023] Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the attached drawings. Embodiments of the present invention are provided to more completely explain the technical idea of ​​the present invention to those skilled in the art. The following embodiments may be modified in various different forms, and the scope of the technical idea of ​​the present invention is not limited to the following embodiments. Rather, these embodiments are provided to more faithfully and completely convey the technical idea of ​​the present invention to those skilled in the art. Like reference numerals throughout this specification denote like elements. Furthermore, various elements and areas in the drawings are schematically drawn. Therefore, the technical idea of ​​the present invention is not limited by the relative sizes or intervals drawn in the attached drawings.

[0024] In recent years, the automotive industry has seen a growing interest in lightweight vehicle bodies utilizing ultra-high-strength steels to simultaneously meet crashworthiness and fuel efficiency regulations. The steel industry is actively researching ultra-high-strength steels to meet these demands from automotive customers. Quenching and Partitioning (Q&P) heat treatment technology, developed to simultaneously secure high strength and high ductility in automotive steels, suppresses the formation of carbide precipitates from carbon released from martensite during quenching and promotes the diffusion of carbon into the retained austenite structure through partitioning. This carbon redistribution stabilizes the retained austenite structure even at room temperature, ultimately ensuring high ductility from the retained austenite structure and high strength from the martensite structure. These Q&P steel plates contain a large amount of silicon (Si) compared to general steel to inhibit the movement of iron (Fe) atoms and suppress the formation of carbide precipitates within the structure. They also contain a large amount of austenite-stabilizing alloying elements such as carbon (C) and manganese (Mn) to increase the volume fraction of stabilized retained austenite structure and improve TRIP (Transformation-Induced Plasticity) behavior.

[0025] Meanwhile, it is obvious that the technical idea of ​​the present invention can be applied to the above-described Q&P steel plate, but it is also obvious that the technical idea of ​​the present invention can be widely applied to various steel plates, and is not limited to the application only to the Q&P steel plate.

[0026] In the present invention, in order to overcome the limitations of the mechanical properties of existing DP steel and TRIP steel, the main matrix of TRIP steel was replaced with tempered martensite and bainite instead of ferrite and inevitably formed fresh martensite, thereby developing a high-strength, high-formability, automotive galvanized steel sheet that can secure high strength and appropriate elongation.

[0027] Specifically, in the present invention, the ratio of austenite phase transformation that may inevitably occur during heat treatment of a QP steel or TRIP steel plating material is reduced through composition control and heat treatment control. Specifically, the composition system is configured by controlling the total amount and ratio of silicon (Si) and aluminum (Al) alloy amounts, and by maintaining the section before reheating after rapid cooling in the manufacturing process, auto tempering at low temperatures and carbon diffusion into austenite are additionally induced compared to the existing heat treatment process, thereby controlling the ratio of unnecessary austenite phase transformation during reheating and alloying heat treatment, thereby providing a plating steel sheet having a target material and a method for manufacturing the same are described.

[0028] The technical idea of ​​the present invention can be applied to a 1.0 GPa grade coated steel sheet having a tensile strength of 980 MPa or more and a method for manufacturing the same, a 1.2 GPa grade coated steel sheet having a tensile strength of 1180 MPa or more and a method for manufacturing the same, or a 1.5 GPa grade coated steel sheet having a tensile strength of 1480 MPa or more and a method for manufacturing the same.

[0029] A method for manufacturing a plated steel sheet according to one embodiment of the present invention sequentially includes the steps of: providing a hot-rolled steel sheet; cold-rolling the hot-rolled steel sheet to provide a cold-rolled steel sheet; annealing the cold-rolled steel sheet; slowly cooling the annealed steel sheet at a first cooling rate; rapidly cooling the slowly cooled steel sheet at a second cooling rate greater than the first cooling rate; reheating the rapidly cooled steel sheet to maintain it at a predetermined temperature range; performing a plating process using the reheated steel sheet as a base steel sheet to form a plating layer on the base steel sheet; and an alloying heat treatment step of maintaining the base steel sheet and the plating layer at a predetermined temperature range and then cooling them to room temperature.

[0030] Hereinafter, a method for manufacturing an ultra-high-strength plated steel sheet having excellent formability and weldability according to one embodiment of the present invention will be described in detail.

[0031] Hot rolled steel sheet provision stage

[0032] A plated steel sheet according to one embodiment of the present invention comprises a base steel sheet (base material) and a plated layer formed on the base steel sheet. The roles and contents of exemplary components included in the base steel sheet are described below. In this case, the content of all component elements is expressed in weight percent.

[0033] Carbon (C): 0.1~0.4%

[0034] Carbon is the most important alloying element in steelmaking, primarily serving to strengthen and stabilize austenite. A high carbon concentration within austenite enhances austenite stability, facilitating the acquisition of adequate austenite for material improvement. Carbon content below 0.1% makes it difficult to achieve the desired yield strength and elongation. Carbon content exceeding 0.4% can lead to a decrease in weldability due to the increased carbon equivalent. Therefore, a carbon content of 0.1% to 0.4% of the total weight of the steel sheet is recommended.

[0035] Meanwhile, the composition range of carbon described above can be further subdivided to secure the necessary properties and microstructure depending on the product group of the galvanized steel sheet to be implemented.

[0036] i) In order to realize a 1.0 GPa grade coated steel sheet having properties such as yield strength: 600 to 760 MPa, tensile strength: 980 MPa or more, and total elongation: 21% or more, the carbon content may be 0.1% to 0.3% of the total weight of the base steel sheet, and strictly speaking, 0.1% to 0.25%.

[0037] ii) In order to realize a 1.2 GPa grade coated steel sheet having properties such as yield strength: 850 to 1070 MPa, tensile strength: 1180 MPa or more, and total elongation: 14% or more, the carbon content may be 0.1% to 0.3% of the total weight of the base steel sheet, and strictly speaking, 0.1% to 0.25%.

[0038] iii) In order to realize a 1.5 GPa grade coated steel sheet having properties such as yield strength: 1000 to 1350 MPa, tensile strength: 1480 MPa or more, and total elongation: 14% or more, the carbon content may be 0.2% to 0.4% of the total weight of the steel sheet, and strictly speaking, may be more than 0.25% and less than 0.4%.

[0039] Manganese (Mn): 1.5~4.0%

[0040] Manganese is an austenite stabilizing element. As manganese is added, the martensite transformation start temperature, Ms, gradually decreases, which can have the effect of increasing the retained austenite fraction during the continuous annealing heat treatment process. If the manganese content is less than 1.5%, the effect of adding manganese is insufficient. If the manganese content exceeds 4.0%, the carbon equivalent increases, which significantly reduces weldability, and oxides (MnO) are formed on the steel sheet surface during the process, which can lead to a decrease in plating properties due to the wettability of the corresponding part. Therefore, the manganese content is preferably 1.5% to 4.0% of the total weight of the base steel sheet.

[0041] Meanwhile, the composition range of manganese described above can be further subdivided to secure the necessary properties and microstructure depending on the product group of the galvanized steel sheet to be implemented.

[0042] i) In order to realize a 1.0 GPa grade coated steel sheet having properties such as yield strength: 600 to 760 MPa, tensile strength: 980 MPa or more, and total elongation: 21% or more, the manganese content may be 1.5% or more and 3.0% or less of the total weight of the base steel sheet, and strictly speaking, may be 1.5% or more and less than 2.5%.

[0043] ii) In order to realize a 1.2 GPa grade coated steel sheet having properties such as yield strength: 850 to 1070 MPa, tensile strength: 1180 MPa or more, and total elongation: 14% or more, the manganese content may be 2.0% or more and 4.0% or less of the total weight of the base steel sheet, and strictly speaking, may be 2.5% or more and 4.0% or less.

[0044] iii) In order to realize a 1.5 GPa grade coated steel sheet having properties such as yield strength: 1000 to 1350 MPa, tensile strength: 1480 MPa or more, and total elongation: 14% or more, the manganese content may be 1.5% or more and 4.0% or less of the total weight of the base steel sheet, and strictly speaking, 2.0% or more and 4.0% or less.

[0045] Silicon (Si): 0% or more and 2.0% or less

[0046] Silicon is an element that suppresses the formation of carbides (e.g., Fe3C) in ferrite and increases the activity of carbon, thereby increasing the diffusion rate of austenite. Silicon is also well known as a ferrite stabilizing element, and is known as an element that increases ductility by increasing the ferrite fraction during cooling. If the silicon content exceeds 2.0%, oxides (SiO2) may be formed on the surface of the base steel sheet during the process, which may result in a decrease in plating properties due to poor wettability in the corresponding part. In addition, when the plated steel sheet is subjected to resistance spot welding, the plating layer may melt, and the molten zinc metal may penetrate toward the interface of the residual austenite present on the surface of the base steel sheet, causing liquid metal embrittlement (LME), which is a phenomenon in which brittleness is caused. According to one embodiment of the present invention, since the base steel sheet contains silicon, according to the above-described contents, the silicon content is preferably more than 0 and 2.0% or less of the total weight of the base steel sheet.

[0047] However, according to another embodiment of the present invention, the steel plate may contain silicon.

[0048] Accordingly, in the present invention, the content of silicon may be 0% or more and 2.0% or less of the total weight of the steel plate. However, even if silicon (Si) is not intentionally added, a very small amount may be detected.

[0049] Aluminum (Al): 0 to 2.0%

[0050] Aluminum plays a role in reducing the austenite transformation fraction that occurs during temperature rise during annealing. Adding aluminum can reduce material dispersion by reducing the change in the abnormal structure fraction during temperature rise.

[0051] Meanwhile, next-generation steel plates such as Q&P steel plates may contain a large amount of silicon (Si) compared to general steel plates to inhibit the movement of iron (Fe) atoms and suppress the formation of carbide precipitates within the structure. However, high silicon content may make the steel susceptible to liquid metal embrittlement (LME), so silicon can be reduced. In this case, aluminum may be added to suppress phase transformation during reheating and alloying heat treatment. In other words, suppressing the phase transformation of residual austenite during alloying heat treatment after the formation of the plating layer during the manufacturing process of the plated steel plate is effective in securing elongation. The present invention proposes an alloy design that implements a high aluminum content and a low silicon content.

[0052] However, if the aluminum content exceeds 2.0%, the annealing temperature for securing an ideal structure may increase excessively, which may reduce mass productivity, and the formation of oxide foreign substances during annealing may increase surface defects such as dents. In addition, it may cause an increase in steelmaking inclusions and surface oxidation during annealing. Therefore, the aluminum content is preferably more than 0 and less than 2.0% of the total weight of the base steel sheet.

[0053] Furthermore, the aluminum content may be, strictly speaking, 0.5% to 2.0% of the total weight of the base steel sheet, and more strictly, more than 0.5% to 2.0% of the total weight of the base steel sheet. In this case, since the carbides formed during the alloying process after plating are small, the chemical stability within austenite is improved, and thus the phase transformation from austenite to pearlite or fresh martensite is relatively small during the final cooling process after the alloying heat treatment, so that the reduction rate of retained austenite can be managed low. In addition, for the plated steel sheet, immediately after spot welding, in a region within 0.1㎛ in the direction of the base steel sheet from the interface between the base steel sheet and the plating layer, the volume fraction of the gamma (Γ) phase, which is an alloy phase, may be 50% or more, and the ratio of the volume fraction of the gamma (Γ) phase, which is an alloy phase, to the volume fraction of the liquid pure zinc (Zn) phase may be implemented to be 1.5 or more. In addition, when conducting a glib evaluation, the strain energy from the point where the ultimate tensile strength (UTS) appears to the point where the fracture occurs can be reduced by 70% or less for the coated steel sheet, which is a plated material, based on the base steel sheet, which is a non-plated material. In other words, it can be confirmed that the strain energy from the point of the highest stress to the point of fracture in the tensile test of the coated steel sheet is 30% or more of the strain energy from the point of the highest stress to the point of fracture in the tensile test of the base steel sheet. The liquid metal embrittlement (LME) characteristics of the QP steel or TRIP steel plating material can be improved through strict control of the aluminum (Al) alloy content described above. According to the strict control of the aluminum (Al) alloy content described above, the amount of aluminum (Al) diffusion to the interface between the plating layer and the steel sheet changes, and accordingly, the phase stability in the interface region can be changed, which can secure liquid metal embrittlement (LME) resistance.That is, for the plated steel sheet, the stability of the Γ phase increases due to changes in the interface composition and phase stability caused by the Al interface diffusion, and the fraction of the interface liquid Zn decreases due to the decrease in the stability of the liquid Zn, so that the amount of penetration into the base steel sheet is reduced due to the reduction in the interface liquid Zn, and thus liquid metal embrittlement (LME) resistance can be secured.

[0054] The sum of silicon (Si) and aluminum (Al) content: 2.0% or less, and the ratio of silicon (Si) content to aluminum (Al) content: less than 4.0.

[0055] In the present invention, the rate of austenite phase transformation that can inevitably occur during heat treatment of QP steel or TRIP steel plating materials is reduced through composition control. For example, by configuring the composition system through control of the total amount and ratio of silicon (Si) and aluminum (Al) alloy amounts, auto-tempering at low temperatures and additional carbon diffusion into austenite are induced, thereby controlling the rate of unnecessary austenite phase transformation during reheating and alloying heat treatment, thereby realizing a plating steel sheet having the desired material.

[0056] Under the condition that the sum of the silicon (Si) content and the aluminum (Al) content in the steel sheet is 2.0% or less, and the ratio of the silicon (Si) content and the aluminum (Al) content is less than 4.0, the unnecessary austenite phase transformation (austenite → pearlite, fresh martensite) ratio during reheating and alloying heat treatment can be controlled to 30% or less.

[0057] In contrast, when the sum of the silicon (Si) content and the aluminum (Al) content exceeds 2.0%, liquid metal embrittlement (LME), a phenomenon in which the plating layer melts during resistance spot welding on the plated steel sheet and molten zinc penetrates toward the interface of the residual austenite existing on the surface of the base steel sheet, causing brittleness, may occur significantly. In addition, the annealing temperature for securing an abnormal structure may increase excessively, which may reduce mass productivity. In addition, surface defects such as dents may increase as oxide foreign substances are formed during annealing, and this may cause an increase in steelmaking inclusions and surface oxidation during annealing.

[0058] Meanwhile, even under the condition that the sum of the silicon (Si) content and the aluminum (Al) content in the base steel sheet of the 1.0 GPa grade galvanized steel sheet is 2.0% or less, if the ratio of the silicon (Si) content and the aluminum (Al) content is 4.0 or more, the unnecessary austenite phase transformation (austenite → pearlite, fresh martensite) ratio during reheating and alloying heat treatment cannot be suppressed to 30% or less, so a problem may arise in which the total elongation is implemented to be less than 21%.

[0059] Meanwhile, even under the condition that the sum of the silicon (Si) content and the aluminum (Al) content in the base steel sheet of the 1.2 GPa or 1.5 GPa grade plated steel sheet is 2.0% or less, if the ratio of the silicon (Si) content and the aluminum (Al) content is 4.0 or more, the unnecessary austenite phase transformation (austenite → pearlite, fresh martensite) ratio during reheating and alloying heat treatment cannot be suppressed to 30% or less, so a problem may arise in which the total elongation is implemented to be less than 14%.

[0060] Phosphorus (P): 0% or more to 0.02% or less

[0061] Phosphorus can play a role similar to silicon in steel. However, if phosphorus is added in amounts exceeding 0.02% of the total weight of the steel sheet, it can reduce weldability and increase brittleness, resulting in material deterioration. Therefore, it is recommended to limit the phosphorus content to 0.02% or less of the total weight of the steel sheet.

[0062] Sulfur (S): 0% or more to 0.005% or less

[0063] Sulfur is an unavoidable element in steel manufacturing. It impairs the steel's toughness and weldability, and combines with manganese (Mn) to form MnS, thereby reducing its corrosion resistance and impact properties. Therefore, it is recommended that the sulfur content be limited to 0.005% or less of the total weight of the steel sheet.

[0064] Nitrogen (N): 0 to 0.006%

[0065] Nitrogen is an element that inevitably occurs during steel manufacturing and degrades aging resistance. Therefore, it is desirable to minimize its presence. Therefore, it is desirable to limit the nitrogen content to 0.006% or less of the total weight of the steel sheet.

[0066] Boron (B): 0 to 0.003%

[0067] Boron is a quenching element in steel. It segregates at grain boundaries during cooling, inhibiting the formation of ferrite and functioning as a grain-boundary strengthening element. However, if the boron content exceeds 0.003%, the strength may increase excessively, and the formation of nitrides such as BN may reduce high-temperature ductility. In the case of plating materials, plating peeling may occur. Therefore, it is preferable to add boron in an amount exceeding 0 and not exceeding 0.003% of the total weight of the steel sheet.

[0068] The remaining component of the base steel sheet, which is the base material for the above-mentioned galvanized steel sheet, is iron (Fe). However, during the typical steelmaking process, unintended impurities from raw materials or the surrounding environment can inevitably be mixed in, and thus cannot be excluded. Since these impurities are readily apparent to anyone skilled in the typical manufacturing process, their full content is not specifically addressed in this specification.

[0069] In the manufacturing method according to the present invention, the semi-finished product subject to the hot rolling and cold rolling processes may be, for example, a slab. The slab in semi-finished form can be obtained through a continuous casting process after obtaining molten steel of a predetermined composition through a steelmaking process.

[0070] A step of forming a hot-rolled steel sheet by applying a hot rolling process to the above steel material is performed. Since the above steel material is a high-alloy steel, edge cracking and rolling load must be minimized to ensure mass production. Therefore, the rolling finishing temperature and coiling temperature can be set to a high temperature range.

[0071] The above steel is reheated at a reheating temperature (Slab Reheating Temperature, SRT) ranging from, for example, 1150°C to 1250°C. This reheating can cause re-dissolution of components segregated during casting and re-dissolution of precipitates. If the reheating temperature is lower than 1150°C, a problem of a rapid increase in hot rolling load may occur. If the reheating temperature exceeds 1250°C, slab warpage may make it difficult to charge and discharge from the furnace, and coarsening of initial austenite grains may make it difficult to secure the strength of the final produced steel sheet. The reheating temperature may vary depending on the steel.

[0072] Next, the reheated steel is hot-rolled, and hot-rolling can be performed at a finish delivery temperature (FDT) of, for example, 800°C to 950°C. If the finish delivery temperature exceeds 950°C, there is a concern that the quality of the steel sheet may deteriorate due to the occurrence of surface scale on the steel sheet. In addition, if the finish delivery temperature is less than 800°C, it may cause an increase in rolling load and a decrease in productivity. The finish delivery temperature may vary depending on the steel sheet.

[0073] Next, the hot-rolled steel is cooled at a cooling rate of 10 to 30°C / s and then coiled at a coiling temperature (CT) ranging from, for example, 400°C to 650°C. The coiling temperature may vary depending on the steel. If the coiling temperature exceeds 650°C, an undesirable internal oxidation layer may occur in the hot-rolled steel sheet or the coiled hot-rolled coil. Since the internal oxidation of the hot-rolled coil thus coiled has deviations, it may be difficult to uniformly control the thickness of the internal oxidation layer. If the coiling temperature is less than 400°C, an undesirable low-temperature structure may be formed.

[0074] Mountaineering stage

[0075] Meanwhile, in a method for manufacturing a plated steel sheet according to one embodiment of the present invention, a softening heat treatment step and / or a pickling step may be sequentially performed after performing a hot rolling process and before performing a cold rolling process.

[0076] However, in the method for manufacturing a plated steel sheet with excellent weldability according to one embodiment of the present invention, the application of the softening heat treatment step may be selective depending on the steel type or target strength. For example, in the case of a 1.2 GPa-grade plated steel sheet having a target tensile strength of 1180 MPa or more after cold rolling / annealing, or a 1.5 GPa-grade plated steel sheet having a target tensile strength of 1480 MPa or more, the softening heat treatment step may be performed, and in the case of a 1.0 GPa-grade plated steel sheet having a target tensile strength of 980 MPa after cold rolling / annealing, the softening heat treatment step may not be performed.

[0077] In the softening heat treatment step, the hot-rolled steel sheet is subjected to softening heat treatment to soften the material, thereby alleviating the problems of reduction ratio load and shape defects during subsequent cold rolling. In other words, softening heat treatment softens the hot-rolled steel sheet to ensure cold-rollability for the efficiency of the cold rolling operation. If the strength of the hot-rolled steel sheet is high, problems such as thickness hunting and shape defects may occur during cold rolling. However, in the case of the ultra-high-strength steel softening heat treatment process, it is applied to hot-rolled coils that still have scale, not cold-rolled coils. Therefore, measures to respond to changes in surface properties due to the high-temperature reaction of the scale during the softening heat treatment are necessary. In general, ultra-high-strength steels containing large amounts of Si and Mn are known to form internal oxides along the grain boundaries of the steel sheet in addition to scale at high temperatures. The oxide layer formed by internal oxidation has poor pickling properties because the main component of the matrix is ​​iron. Therefore, there is a problem that the internal oxide layer cannot be completely removed with the same pickling time as that of general hot-rolled steel sheets, and a long pickling time is required, which reduces productivity. This internal oxidation occurs when the activity of easily oxidized elements such as Si and Mn is high and exists under specific oxygen partial pressure conditions. Therefore, when a hot-rolled coil with remaining scale is heat-treated in a high-temperature reducing gas atmosphere, additional internal oxidation occurs due to the oxygen generated during the scale reduction reaction. Specifically, internal oxide, which was not observed in the hot-rolled material, developed unevenly over the entire coil length after the softening heat treatment, and the scale reduction and internal oxidation development behavior may differ depending on the location within the coil. In the outer winding of the softening heat-treated coil, internal oxidation growth due to scale hydrogen reduction reaction was mainly observed, and in the inner winding of the coil, internal oxidation growth due to the scale eutectoid reaction (4FeO → 4Fe + 2O2) was observed.It is believed that this difference in internal oxidation growth behavior is due to the difference in the ease of penetration of the reaction gas depending on the position of the winding coil, and it was understood that the oxygen generated from the scale hydrogen reduction and eutectoid reaction during the softening heat treatment diffuses into the parent material and acts as an internal oxidation reaction material. Since it is desirable for the internal oxidation layer to be formed as uniformly as possible throughout the steel sheet, it is desirable to suppress the internal oxidation layer as much as possible during the winding step and form the internal oxidation layer during the softening heat treatment.

[0078] Considering these aspects, in the method for manufacturing a plated steel sheet of the present invention, the coiling temperature is controlled to 400°C to 650°C, and the softening heat treatment temperature is controlled to 450°C to 650°C. The softening heat treatment may be performed in a batch annealing furnace (BAF) while the hot-rolled steel sheet is coiled, and may be performed in a hydrogen atmosphere. The hot-rolled steel sheet that has undergone the softening heat treatment under the above process conditions may have a softened material and may secure cold-rollability. In addition, since the internal oxide layer formed by the softening heat treatment has a predetermined thickness (for example, a thickness of 10 μm or less), subsequent pickling properties may be secured at the same time.

[0079] When softening heat treatment is applied at a temperature below 450℃, the martensite formed after hot rolling does not recrystallize, but only undergoes tempering, so that supersaturated carbon within the structure forms and spheroidizes in the form of cementite (θ). In this case, the brittleness of martensite may occur, which may lead to safety accidents such as sheet fracture during cold rolling. In other words, if an excessive amount of austenite is formed during softening heat treatment, martensite may be formed during cooling, preventing the strength reduction from being effectively realized.

[0080] In addition, when the softening heat treatment is applied at a temperature exceeding 650℃, the internal oxide layer formed by the softening heat treatment exceeds a predetermined thickness (for example, exceeding a thickness of 10㎛), making it difficult to secure subsequent acid washability. In addition, when the softening heat treatment is applied at a temperature exceeding 650℃, austenite is excessively formed, and martensite is formed during cooling, so the strength reduction is not effectively realized.

[0081] The time for performing the above softening heat treatment may be 1 to 12 hours.

[0082] In the pickling treatment step, a pickling treatment may be performed to clean the hot-rolled steel sheet with acid. By pickling the hot-rolled steel sheet, at least a portion of the internal oxide layer may be removed. The pickling treatment may be performed, for example, at a temperature of 70°C to 90°C, with a hydrochloric acid concentration of 5% to 15%, for example, 20 to 40 seconds. In addition, the pickling treatment may have an inhibitor concentration of 0.1 to 0.5%.

[0083] Cold rolling stage

[0084] A step of forming a cold-rolled steel sheet by applying a cold-rolling process to the above hot-rolled steel sheet is performed. In the case of cold rolling, it is performed to match the thickness of the final produced steel sheet using hot-rolled material.

[0085] In the cold rolling step, the hot-rolled steel sheet that has undergone the above-mentioned pickling treatment can be cold rolled at an average reduction ratio of, for example, 40% to 60%, thereby producing a cold-rolled steel sheet. The microstructure of the cold-rolled steel sheet has an elongated shape of the hot-rolled steel sheet, and the microstructure of the final steel sheet is determined by the subsequent heat treatment.

[0086] Annealing heat treatment stage

[0087] Cold-rolled steel sheets can be annealed in a continuous annealing furnace with a slow cooling section. This annealing is performed to form an austenitic structure. The annealing temperature and time affect the austenite grain size and, therefore, can significantly influence the strength of the cold-rolled steel sheet.

[0088] The above annealing heat treatment is performed at a heating rate of, for example, 1°C / s or higher, for example, at a heating rate in the range of 1°C / s to 10°C / s. If the heating rate is less than 1°C / s, it takes a long time to reach the target annealing heat treatment temperature, which reduces production efficiency and may increase the size of crystal grains.

[0089] During these heating and annealing heat treatment steps, the cold-rolled structure is reversely transformed into austenite.

[0090] If the annealing heat treatment time exceeds 130 seconds, the heat treatment efficiency may decrease. If the annealing heat treatment time is less than 60 seconds, the annealing heat treatment effect may be insufficient. Therefore, the annealing heat treatment time is in the range of 60 to 130 seconds. As the annealing heat treatment time increases, it affects the coarsening due to austenite grain growth, similar to the annealing heat treatment temperature.

[0091] Meanwhile, the temperature range of the annealing heat treatment may be subdivided to secure the necessary properties and microstructure depending on the product group of the plated steel sheet to be implemented in the present invention.

[0092] i) In the method for manufacturing a 1.0 GPa grade coated steel sheet, the step of performing annealing heat treatment may include a step of maintaining the cold rolled steel sheet in a temperature range of 800 to 850°C for 60 to 130 seconds. That is, the annealing heat treatment temperature may be set in an ideal temperature range where austenite and ferrite coexist. The annealing heat treatment may be performed, for example, at an austenite and ferrite ideal temperature of Ae1 to Ae3, for example, at a temperature in the range of 800°C to 850°C, for a time range of 60 to 130 seconds. Within the steel composition range of the 1.0 GPa grade coated steel sheet, when the annealing heat treatment temperature is less than 800°C, austenite cannot be formed to create tempered martensite, which is the final structure. When the annealing heat treatment temperature exceeds 850°C, austenite grains may become coarse, which may reduce strength.

[0093] ii) In the method for manufacturing a 1.2 GPa grade coated steel sheet, the step of performing annealing heat treatment may include a step of maintaining the cold rolled steel sheet in a temperature range of 800 to 850°C for 60 to 130 seconds. That is, the annealing heat treatment temperature may be set in an ideal temperature range where austenite and ferrite coexist. The annealing heat treatment may be performed, for example, by maintaining the temperature in an austenite and ferrite ideal range of Ae1 to Ae3, for example, in a temperature range of 800°C to 850°C, for a time range of 60 to 130 seconds. Within the steel composition range of the 1.2 GPa grade coated steel sheet, when the annealing heat treatment temperature is less than 800°C, austenite cannot be formed to create tempered martensite, which is the final structure. When the annealing heat treatment temperature exceeds 850°C, austenite grains may become coarse, which may reduce strength.

[0094] iii) In the method for manufacturing a 1.5 GPa grade coated steel sheet, the step of performing annealing heat treatment may include a step of maintaining the cold rolled steel sheet in a temperature range of 850 to 900°C for 60 to 130 seconds. That is, the annealing heat treatment temperature may be set in a single-phase temperature range where austenite exists and ferrite does not exist. The annealing heat treatment may be performed, for example, at a temperature of Ae3 or higher, an austenite single-phase region temperature, for example, at a temperature in the range of 850°C to 900°C, for a time in the range of 60 to 130 seconds. Within the steel composition range of the 1.5 GPa grade coated steel sheet, when the annealing heat treatment temperature is less than 850°C, austenite cannot be formed to create a final structure, tempered martensite. When the annealing heat treatment temperature exceeds 900°C, austenite grains may become coarse, which may reduce strength.

[0095] In the annealing heat treatment step, the dew point temperature of the atmospheric gas inside the annealing furnace is controlled to be -10°C or higher and +20°C or lower. The atmospheric gas inside the annealing furnace may be, for example, a mixed gas of 90% nitrogen and 10% oxygen. By using a humidifier attached to the outer wall of the annealing furnace to spray pure H2O into the annealing furnace, the dew point (dew point) of the heat treatment atmospheric gas can be controlled to be -10°C or higher and +20°C or lower. Through the annealing heat treatment having the above-described dew point temperature range, the base steel sheet constituting the plated steel sheet of the present invention can undergo a decarburization reaction on the surface of the steel sheet.

[0096] Multi-stage cooling stage

[0097] The above-mentioned annealed cold-rolled steel sheet is subjected to multi-stage cooling. The cooling step may be performed in two steps. That is, the multi-stage cooling includes a step of slowly cooling the annealed steel sheet at a first cooling rate; and a step of rapidly cooling the slowly cooled steel sheet to a temperature below a martensite transformation initiation temperature (Ms) at a second cooling rate that is higher than the first cooling rate. The first cooling rate is lower than 20°C / s, the second cooling rate is higher than 20°C / s (strictly higher than 40°C / s), and the slow cooling end temperature of the slow cooling step may be 650 to 750°C, and the rapid cooling end temperature of the rapid cooling step may be 200 to 300°C. The rapid cooling step is characterized in that after reaching the rapid cooling end temperature, the rapid cooling end temperature is maintained for 20 to 60 seconds. The multi-stage cooling step will be described in detail below.

[0098] First, the cold-rolled steel sheet subjected to the annealing heat treatment is first cooled by slow cooling to a temperature range that suppresses ferrite transformation, for example, a first cooling end temperature in the range of 650°C to 750°C, at a first cooling rate of, for example, 3°C / s to 20°C / s. By controlling the cooling rate from the annealing temperature, the formation of ferrite with a high manganese (Mn) concentration can be suppressed, and the amount of ferrite formed can be suppressed. When the average cooling rate is slower than 3°C / s, ferrite with a high manganese (Mn) concentration may be formed during cooling, which may deteriorate bendability or reduce strength.

[0099] Next, a second cooling step is performed to rapidly cool the first-cooled cold-rolled steel sheet to a second cooling end temperature below the martensite transformation initiation temperature (Ms) at a second cooling rate of, for example, more than 20°C / s and less than or equal to 100°C / s. The second cooling end temperature is a temperature below the Ms temperature and may range from 200°C to 300°C, for example.

[0100] In the above second cooling step, cooling is performed at a rapid cooling rate, so that transformation of ferrite, pearlite or bainite is suppressed, and a portion of austenite can be transformed into martensite.

[0101] If the cooling rate in the secondary cooling step is less than 20℃ / s, the fraction of austenite after cooling is too high, making it difficult to secure the stability of the retained austenite. In addition, even if the bainite transformation structure increases, the fraction of martensite is small, which may result in a decrease in strength. In addition, some martensite structures increase internal stress, which increases the rate of bainite nucleation, so that the bainite transformation proceeds rapidly even at low temperatures below the martensite transformation initiation temperature (Ms).

[0102] The cold-rolled steel sheet subjected to the second cooling can be maintained at the second cooling end temperature for a predetermined period of time, for example, 20 to 60 seconds. During the period in which the second cooling end temperature is maintained, some of the austenite is transformed into martensite, and precipitates such as metal carbides may be formed within the martensite formed in the second cooling step. If excessive precipitates are formed within the martensite during the period in which the second cooling end temperature is maintained, the elongation of the final product may exhibit a relatively low value. Therefore, the period in which the second cooling end temperature is maintained should be within a range that suppresses the formation of excessive precipitates, and thus, in the present invention, the period in which the precipitates are maintained is 20 to 60 seconds.

[0103] In the present invention, the ratio of austenite phase transformation that may inevitably occur during heat treatment of a QP steel or TRIP steel plating material is reduced by controlling the total amount and ratio of silicon (Si) and aluminum (Al) alloy amounts to configure a component system, and then maintaining the temperature in the section before reheating after the secondary cooling, which is the rapid cooling step described above. For example, by securing the secondary cooling end temperature maintenance section for 20 to 60 seconds after the secondary cooling, which is the rapid cooling step described above, auto tempering at low temperatures and carbon diffusion into austenite are additionally induced, thereby controlling the unnecessary austenite phase transformation ratio during reheating and alloying heat treatment, and thereby implementing a plating steel sheet having a target material.

[0104] i) In the manufacturing method of a 1.0 GPa grade plated steel sheet, if the second cooling end temperature maintenance period after the above-mentioned second cooling, which is the rapid cooling step, is less than 20 seconds, the unnecessary austenite phase transformation ratio (austenite → pearlite, fresh martensite) during reheating and alloying heat treatment cannot be suppressed to 30% or less, and thus the elongation (total elongation) is realized to be less than 21%. If the second cooling end temperature maintenance period after the above-mentioned second cooling, which is the rapid cooling step, exceeds 60 seconds, if metal carbides are excessively precipitated within the martensite, it becomes difficult to secure an elongation of 21% or more in the final product.

[0105] ii) In the manufacturing method of a 1.2 GPa grade plated steel sheet, if the secondary cooling end temperature maintenance period after the secondary cooling, which is the rapid cooling step described above, is less than 20 seconds, the unnecessary austenite phase transformation ratio (austenite → pearlite, fresh martensite) during reheating and alloying heat treatment cannot be suppressed to less than 30%, so the elongation (total elongation) is implemented to be less than 14%. If the secondary cooling end temperature maintenance period after the secondary cooling, which is the rapid cooling step described above, exceeds 60 seconds, if metal carbides are excessively precipitated within the martensite, it becomes difficult to secure an elongation of 14% or more in the final product.

[0106] iii) In the manufacturing method of a 1.5 GPa grade plated steel sheet, if the second cooling end temperature maintenance period after the above-mentioned second cooling, which is the rapid cooling step, is less than 20 seconds, the unnecessary austenite phase transformation ratio (austenite → pearlite, fresh martensite) during reheating and alloying heat treatment cannot be suppressed to less than 30%, so the elongation (total elongation) is realized to be less than 14%. If the second cooling end temperature maintenance period after the above-mentioned second cooling, which is the rapid cooling step, exceeds 60 seconds, if metal carbides are excessively precipitated within the martensite, it becomes difficult to secure an elongation of 14% or more in the final product.

[0107] Reheating heat treatment step

[0108] A reheating step is performed to maintain the above-described rapidly cooled steel sheet in a temperature range from a martensite transformation initiation temperature (Ms) to a bainite transformation initiation temperature (Bs). That is, the multi-stage cooled cold-rolled steel sheet is reheated at a heating rate of, for example, 40°C / s or more, and is maintained at a temperature in the range of, for example, 350°C to 470°C for a time in the range of, for example, 20 seconds to 50 seconds, thereby performing a reheating heat treatment.

[0109] In the reheating heat treatment (partitioning heat treatment) step, carbon diffuses and concentrates within the retained austenite, thereby stabilizing the retained austenite. In addition, some of the austenite may undergo martensitic transformation. The martensitic transformation may refine the shape of the retained austenite after rapid cooling, thereby contributing to the stabilization of the retained austenite. If the partitioning heat treatment temperature is less than 350°C, the partitioning effect described above may be insufficient. If the reheating heat treatment temperature exceeds 470°C, the size of the carbides may become coarser, resulting in a decrease in strength.

[0110] If the reheating heat treatment holding time is less than 20 seconds, it may be difficult to obtain a stable partitioning effect. If the reheating heat treatment holding time exceeds 50 seconds, the heat treatment efficiency may decrease, and the carbide size may increase, resulting in a decrease in strength.

[0111] Plating process steps

[0112] A step of continuously performing a plating process using the reheated steel plate as a base steel plate to form a plating layer on the base steel plate is performed. The reheated steel plate may be immersed in a molten plating bath to perform molten plating to form a plating layer. The plating bath may be a molten zinc plating bath, and the entry temperature of the plating bath may be, for example, 460°C. The plating deposition amount is 40 to 200 g / m on both sides. 2 , and the plating layer thickness can be 10 to 30㎛.

[0113] Alloying heat treatment stage

[0114] The above-mentioned base steel sheet and the plating layer may be subjected to an alloying heat treatment step of maintaining the temperature range of 480 to 560°C. The time for maintaining the temperature range of 480 to 560°C in the alloying heat treatment step may be 15 to 45 seconds. The plated steel sheet realized through the alloying heat treatment after the above-mentioned zinc hot-dip plating is an alloying hot-dip galvanized steel sheet. An Fe-Zn alloy phase may be formed within the plating layer through the alloying heat treatment.

[0115] When the alloying heat treatment temperature is less than 480°C, the alloying heat treatment effect described above may be insufficient, making it difficult to form an Fe-Zn alloy phase within the plating layer. When the alloying heat treatment temperature exceeds 560°C, transformation-induced plasticity may not be expressed due to the decomposition of residual austenite in addition to the tempering effect of the matrix structure as the alloying progresses, which may result in a decrease in strength.

[0116] Meanwhile, even if the time for which the alloying heat treatment is maintained in the temperature range of 480 to 560°C is less than 15 seconds, the above-described alloying heat treatment effect may be insufficient, making it difficult to form an Fe-Zn alloy phase within the plating layer. In addition, if the time for which the alloying heat treatment is maintained in the temperature range of 480 to 560°C exceeds 45 seconds, the size of carbides may become coarser, resulting in a decrease in strength.

[0117] Meanwhile, the alloying heat treatment step may include a step of maintaining the temperature in the range of 480 to 560°C for 15 to 45 seconds and then cooling to room temperature at a third cooling rate. The third cooling rate may be greater than the first cooling rate of the above-described slow cooling step and less than the second cooling rate of the above-described rapid cooling step.

[0118] In the present invention, it was intended to secure the material (particularly, elongation) of the steel plate by suppressing to the maximum extent the phenomenon in which austenite undergoes phase transformation into at least one of pearlite and fresh martensite during the alloying heat treatment step, and to achieve this, it was described above that the composition system can be formed by controlling the total amount and ratio of silicon (Si) and aluminum (Al) alloy amounts, and then maintaining the composition in the section before reheating after the rapid cooling step, which is the secondary cooling described above.

[0119] A plated steel sheet manufactured by the above-described manufacturing method is a plated steel sheet comprising a base steel sheet containing, in wt%, carbon (C): 0.1 to 0.4%, silicon (Si): more than 0% and 2.0% or less, manganese (Mn): 1.5 to 4.0%, aluminum (Al): more than 0% and 2.0% or less, phosphorus (P): more than 0% and 0.02% or less, sulfur (S): more than 0% and 0.005% or less, nitrogen (N): more than 0% and 0.006% or less, boron (B): more than 0% and 0.003% or less, and the remainder including iron (Fe) and other inevitable impurities; and a plating layer on the base steel sheet; characterized in that the sum of the content of silicon (Si) and the content of aluminum (Al) in the base steel sheet is 2.0% or less, and the ratio of the content of silicon (Si) to the content of aluminum (Al) is less than 4.0.

[0120] Depending on the product group of the galvanized steel sheet to be implemented, the composition range described above may be further limited, and the properties and microstructure implemented accordingly are as follows.

[0121] i) 1.0GPa grade galvanized steel sheet

[0122] The above 1.0 GPa grade coated steel sheet is a coated steel sheet comprising a base steel sheet containing, in wt%, carbon (C): 0.1 to 0.3% (strictly, 0.1 to 0.25%), silicon (Si): more than 0% and 2.0% or less, manganese (Mn): 1.5 to 3.0% (strictly, 1.5 to 2.5%), aluminum (Al): more than 0 to 2.0% or less, phosphorus (P): more than 0 to 0.02% or less, sulfur (S): more than 0 to 0.005% or less, nitrogen (N): more than 0 to 0.006% or less, boron (B): more than 0 to 0.003% or less, and the remainder iron (Fe) and other unavoidable impurities; and a coated steel sheet on the base steel sheet.

[0123] The above-mentioned galvanized steel sheet has properties such as a yield strength of 600 to 760 MPa, a tensile strength of 980 MPa or more, and a total elongation of 21% or more. For example, the above-mentioned galvanized steel sheet has properties such as a tensile strength of 980 MPa or more and less than 1180 MPa, and a total elongation of 21% or more and less than 30%.

[0124] The final microstructure of the base steel sheet among the above-mentioned plated steel sheets is characterized by an area fraction of, ferrite: 30 to 50%, retained austenite: 5 to 20%, the sum of tempered martensite and bainite: 30 to 65%, and the remainder being at least one of pearlite and fresh martensite.

[0125] If the area fraction of ferrite is less than 30%, the workability of the steel sheet becomes poor, and if it exceeds 50%, it may not be easy to achieve a tensile strength of 980 MPa or higher in the steel sheet. On the other hand, if the area fraction of retained austenite is less than 5%, it is difficult to expect the effect of transformation-induced plasticity, and if it exceeds 20%, the proportion of low-temperature structures (tempered martensite and bainite) is relatively low, making it difficult to secure the strength of the steel sheet.

[0126] ii) 1.2 GPa grade galvanized steel sheet

[0127] The above 1.2 GPa grade coated steel sheet is a coated steel sheet comprising a base steel sheet containing, in wt%, carbon (C): 0.1 to 0.3% (strictly, 0.1 to 0.25%), silicon (Si): more than 0% and 2.0% or less, manganese (Mn): 2.0 to 4.0% (strictly, 2.5 to 4.0%), aluminum (Al): more than 0 to 2.0% or less, phosphorus (P): more than 0 to 0.02% or less, sulfur (S): more than 0 to 0.005% or less, nitrogen (N): more than 0 to 0.006% or less, boron (B): more than 0 to 0.003% or less, and the remainder iron (Fe) and other unavoidable impurities; and a coated steel sheet on the base steel sheet.

[0128] The above-mentioned galvanized steel sheet has properties such as a yield strength of 850 to 1070 MPa, a tensile strength of 1180 MPa or more, and a total elongation of 14% or more. For example, the above-mentioned galvanized steel sheet has properties such as a tensile strength of 1180 MPa or more and 1300 MPa or less, and a total elongation of 14% or more and 20% or less.

[0129] The final microstructure of the base steel sheet among the above-mentioned plated steel sheets is characterized by an area fraction of, ferrite: 5 to 25%, retained austenite: 5 to 20%, the sum of tempered martensite and bainite: 30 to 90%, and the remainder being at least one of pearlite and fresh martensite.

[0130] If the area fraction of ferrite is less than 5%, the workability of the steel sheet becomes poor, and if it exceeds 25%, it may not be easy to achieve a tensile strength of 1180 MPa or more in the steel sheet. On the other hand, if the area fraction of retained austenite is less than 5%, it is difficult to expect the effect of transformation-induced plasticity, and if it exceeds 20%, the proportion of low-temperature structures (tempered martensite and bainite) becomes relatively low, making it difficult to secure the strength of the steel sheet.

[0131] iii) 1.5 GPa grade galvanized steel sheet

[0132] The above 1.5 GPa grade coated steel sheet is a coated steel sheet comprising a base steel sheet containing, in wt%, carbon (C): 0.2 to 0.4% (strictly, more than 0.25% to 0.4%), silicon (Si): more than 0% to 2.0%, manganese (Mn): 1.5 to 4.0% (strictly, more than 2.0% to 4.0%), aluminum (Al): more than 0 to 2.0%, phosphorus (P): more than 0 to 0.02% or less, sulfur (S): more than 0 to 0.005% or less, nitrogen (N): more than 0 to 0.006% or less, boron (B): more than 0 to 0.003% or less, and the remainder iron (Fe) and other unavoidable impurities; and a coated steel sheet on the base steel sheet.

[0133] The above-mentioned coated steel sheet has properties such as a yield strength of 1000 to 1350 MPa, a tensile strength of 1480 MPa or more, and a total elongation of 14% or more. For example, the above-mentioned coated steel sheet has properties such as a tensile strength of 1480 MPa or more and 1550 MPa or less, and a total elongation of 14% or more and 17% or less.

[0134] The final microstructure of the base steel sheet among the above-mentioned plated steel sheets is characterized by an area fraction of, ferrite: 0 to 5%, retained austenite: 5 to 35%, the sum of tempered martensite and bainite: 30 to 95%, and the remainder being at least one of pearlite and fresh martensite.

[0135] If the area fraction of ferrite exceeds 5%, it may not be easy to achieve a tensile strength of 1480 MPa or higher in the steel sheet. On the other hand, if the area fraction of retained austenite is less than 5%, it is difficult to expect the effect of transformation-induced plasticity, and if it exceeds 35%, the proportion of low-temperature structures (tempered martensite and bainite) is relatively low, making it difficult to secure the strength of the steel sheet.

[0136] Meanwhile, in the 1.0GPa grade, 1.2GPa grade, and 1.5GPa grade coated steel sheets described above, the pearlite and fresh martensite are the results of decomposition of residual austenite during heat treatment of the QP steel or TRIP steel coating material in the present invention. For example, at least one of the pearlite and fresh martensite may be 30% or less in area fraction. If at least one of the pearlite and fresh martensite exceeds 30% in area fraction, it means that the stability of the residual austenite is low, and since the residual austenite is decomposed, the degree to which transformation-induced plasticity is expressed is weak, so that the elongation characteristics may be inferior. That is, if a large amount of pearlite and fresh martensite are generated in the final cooling stage, the fraction of final retained austenite may decrease, which may adversely affect formability. Therefore, the fraction of retained austenite transformed into pearlite and fresh martensite during the final cooling stage should be controlled to an area ratio of 30% or less. Therefore, it is necessary to secure the stability of austenite in advance before the alloying heat treatment step of the plating material, and the stability of the austenite can be evaluated in the final microstructure by the parameters disclosed in the following equation 1.

[0137] (Formula 1)

[0138] (X Pearlite + X Fresh Martensite ) / (100 - X Ferrite - X Tempered Martensite- X Bainite ) ≤ 0.30

[0139] In the above formula 1, the above X Pearlite is the value of the area fraction of pearlite (unit: %), and the above X Fresh Martensite is the value of the area fraction of fresh martensite (unit: %), and the above X Ferrite is the value of the area fraction of ferrite (unit: %), and X Tempered Martensite is the value of the area fraction of tempered martensite (unit: %), and the above X Bainite is the value of the area fraction of bainite (unit: %).

[0140] In the present invention, in order to overcome the limitations of the mechanical properties of existing DP steel or TRIP steel, the main matrix of TRIP steel was replaced with tempered martensite and bainite instead of ferrite and inevitably formed fresh martensite or pearlite, thereby realizing a high-strength, high-formability, automotive galvanized steel sheet that can secure high strength and appropriate elongation.

[0141] Furthermore, the plated steel sheet of the present invention described above can improve liquid metal embrittlement (LME) characteristics.

[0142] According to one embodiment of the present invention, a plated steel sheet is characterized in that it contains, in wt%, carbon (C): 0.1 to 0.4%, silicon (Si): more than 0% and 2.0% or less, manganese (Mn): 1.5 to 4.0%, aluminum (Al): more than 0% and 2.0% or less, phosphorus (P): more than 0% and 0.02% or less, sulfur (S): more than 0% and 0.005% or less, nitrogen (N): more than 0% and 0.006% or less, boron (B): more than 0% and 0.003% or less, and the remainder includes iron (Fe) and other inevitable impurities, but the sum of the content of silicon (Si) and the content of aluminum (Al) is 2.0% or less, and the ratio of the content of silicon (Si) and the content of aluminum (Al) is less than 4.0; And a plating layer on the above-mentioned base steel plate; and characterized in that the strain energy from the point of highest stress to the point of fracture in a tensile test on the above-mentioned plated steel plate is 30% or more of the strain energy from the point of highest stress to the point of fracture in a tensile test on the above-mentioned base steel plate.

[0143] Figure 1 is a graph showing the results of a Gleeble tensile test on the coated steel sheet and the base steel sheet of the present invention. In order to evaluate the tensile properties of the steel sheet under high temperature conditions such as electric resistance welding, a high temperature deformation simulator, Gleeble equipment, was used. In Figure 1, the horizontal axis represents the strain, which means the deformed length compared to the reference length, and the vertical axis represents the stress received by the specimen, which means the stress per unit area.

[0144] Referring to Figure 1, when a tensile test is performed on a steel plate, it goes through a strain hardening section (YT) that passes the elastic deformation section (OY) and reaches the point (T) where the ultimate tensile strength (UTS), which is the point with the highest stress, appears. Point (Y), which is one end of the elastic deformation section (OY), corresponds to the yield point. After passing the point (T) where the ultimate tensile strength (UTS), which is the point with the highest stress, appears, a section (TB) occurs where deformation continues but the stress decreases. The material behavior phenomenon in this section is called necking. If stress continues to be applied to the specimen after passing the necking section, the specimen will eventually break at the point of fracture. Meanwhile, the area of ​​the region bordering the horizontal axis and the stress curve in the stress-strain graph corresponds to the strain energy.

[0145] According to one embodiment of the present invention, a plated steel sheet is composed of a base steel sheet and a plated layer, and in a tensile test on the plated steel sheet, the strain energy from the point of highest stress to the point of fracture corresponds to the area of ​​a region (ZTC) bordering the stress curve and the horizontal axis from the point (T) where the maximum tensile strength (UTS) appears to the point (C) where the fracture occurs in FIG. 1, and in a tensile test on the base steel sheet, the strain energy from the point of highest stress to the point of fracture corresponds to the area of ​​a region (ZTB) bordering the stress curve and the horizontal axis from the point (T) where the maximum tensile strength (UTS) appears to the point (B) where the fracture occurs in FIG. 1.

[0146] According to one embodiment of the present invention, a plated steel sheet is a plated steel sheet implemented with the above-described alloy composition and manufacturing process, and is characterized in that the strain energy from the point of highest stress to the point of fracture in a tensile test on the plated steel sheet is 30% or more of the strain energy from the point of highest stress to the point of fracture in a tensile test on the base steel sheet. That is, when a glibble evaluation is performed, the strain energy from the point where the maximum tensile strength (UTS) appears to the point where fracture occurs is characterized in that the energy reduction rate for the plated steel sheet, which is a plated material, is 70% or less based on the base steel sheet, which is an unplated material.

[0147] FIG. 2 is a drawing illustrating the results of simulating the movement of the interface between the plating layer and the base steel sheet and the aluminum diffusion pattern immediately after spot welding in a plated steel sheet according to one embodiment of the present invention, and FIG. 3 is a drawing illustrating the change in stability of the interface according to aluminum diffusion in a plated steel sheet according to one embodiment of the present invention.

[0148] Referring to FIGS. 2 and 3, in the case of a plated steel sheet according to an embodiment of the present invention, when subjected to electric resistance spot welding, it can be confirmed that liquid Zn and an alloy phase (Γ phase) are formed due to instantaneous heating and cooling, and at the same time, the interface between the plated layer and the base steel sheet moves, and aluminum (Al), a component contained in the base steel sheet, diffuses toward the interface. It can be understood that the point in time when aluminum (Al) diffuses and the concentration distribution becomes stable is 1.0 second after spot welding.

[0149] Specifically, referring to FIG. 2, the sum of the content of silicon (Si) and the content of aluminum (Al) is 2.0% or less, and the ratio of the content of silicon (Si) and the content of aluminum (Al) is less than 4.0. As a result of the simulation, the aluminum content is evaluated to be 2 to 3 wt% in a region within 0.1 μm in the direction of the base steel sheet from the interface between the base steel sheet and the plating layer in a range that satisfies the composition.

[0150] Referring to Fig. 3, it can be confirmed that the interface composition and phase stability change according to the diffusion of the interface between the base steel plate and the plating layer, and that the stability of the gamma (Γ) phase, which is an alloy phase, increases and the stability of the liquid pure zinc (Zn) phase decreases in the range of the aluminum content (concentration) from C1 (2 wt%) to C2 (3 wt%).

[0151] In a plated steel sheet according to one embodiment of the present invention, immediately after spot welding, in a region within 0.1 μm in the direction of the base steel sheet from the interface between the base steel sheet and the plated layer, the volume fraction of the gamma (Γ) phase, which is an alloy phase, is 50% or more, and the ratio of the volume fraction of the gamma (Γ) phase, which is an alloy phase, to the volume fraction of the liquid pure zinc (Zn) phase is 1.5 or more. The immediately after spot welding can be understood as a section within 1.0 second after spot welding.

[0152] So far, a plated steel sheet and a manufacturing method thereof according to an embodiment of the present invention have been described. In the present invention, the liquid metal embrittlement (LME) characteristics of a QP steel or TRIP steel plating material are improved through component control and process condition control. More specifically, by controlling the total amount and ratio of silicon (Si) and aluminum (Al) alloys to configure a component system, and by performing an appropriate heat treatment process to manufacture a QP / TRIP plated steel sheet of the target material, and then performing spot welding, the amount of aluminum (Al) diffusion to the interface between the plating layer and the steel sheet changes depending on the total amount and ratio of silicon (Si) and aluminum (Al) alloys, and thus the phase stability in the interface region changes, thereby securing liquid metal embrittlement (LME) resistance.

[0153] Experimental example

[0154] Below, preferred experimental examples are presented to aid understanding of the present invention. However, the following experimental examples are provided solely to aid understanding of the present invention, and the present invention is not limited to the following experimental examples.

[0155] Experimental Example 1 (1.0 GPa-grade coated steel plate)

[0156] Table 1 shows the composition (unit: weight%) of the main components of the base steel sheet constituting the plated steel sheet according to the first experimental example of the present invention. In addition, the composition of phosphorus (P): 0.01%, sulfur (S): 0.003%, nitrogen (N): 0.003%, and boron (B): 0.002% was applied identically.

[0157] CSiMnAl[Si] / [Al] <4 Satisfaction Comparative Example 10.21.52-X Comparative Example 20.21.52-X Comparative Example 30.21.52-X Comparative Example 40.21.320.3X Comparative Example 50.21.320.3X Comparative Example 60.21.320.3X Invention Example 10.2120.51O Invention Example 20.2120.51O Invention Example 30.2120.51O Invention Example 40.20.521O Invention Example 50.20.521O Invention Example 60.20.521O Invention Example 70.2-21.5O Invention Example 80.2-21.5O Invention Example 90.2-21.5O Comparative Example 70.20.521O

[0158] Referring to Table 1, Invention Example 1-9 contains, in wt%, carbon (C): 0.1 to 0.3% (strictly, 0.1 to 0.25%), silicon (Si): more than 0% and 2.0% or less, manganese (Mn): 1.5 to 3.0% (strictly, 1.5 to 2.5%), aluminum (Al): more than 0 and 2.0% or less, phosphorus (P): more than 0 and 0.02% or less, sulfur (S): more than 0 and 0.005% or less, nitrogen (N): more than 0 and 0.006% or less, boron (B): more than 0 and 0.003% or less, and satisfies all the composition ranges of iron (Fe), and the sum of the content of silicon (Si) and the content of aluminum (Al) is 2.0% or less, and the ratio of the content of silicon (Si) and the content of aluminum (Al) is less than 4.0. Everyone is satisfied.

[0159] Meanwhile, Invention Example 7-9 is a case where silicon (Si) was not intentionally added. However, in this case, it can be understood that silicon (Si) is contained at 0.01% or less. This is because even if silicon (Si) is not intentionally added, a very small amount can be detected as an inevitable impurity.

[0160] In contrast, Comparative Example 1-6 does not satisfy the condition that the ratio of the content of silicon (Si) to the content of aluminum (Al) is less than 4.0.

[0161] Table 2 shows the process conditions for manufacturing a plated steel sheet according to the first experimental example of the present invention. The unit of the temperature item is ℃, the unit of the speed item is ℃ / s, and the unit of the time and maintenance items is second (s). In the experimental example of the present invention, other process conditions were applied as conditions that satisfied the process conditions described above but had the same single value. For example, the hot rolling process applied the process conditions of reheating temperature: 1200℃, finishing rolling temperature: 900℃, and coiling temperature: 600℃.

[0162] Annealing temperature, Annealing time, Slow cooling rate, Slow cooling temperature, Rapid cooling rate, Rapid cooling temperature, Rapid cooling holding, Reheating rate, Reheating temperature, Reheating holding, Alloying temperature, Alloying holding, Comparative example 1 790 803.475 0 462 153 488 460 30 480 24 Comparative example 2 790 803.475 0 462 153 488 460 30 5 2024 Comparative example 3 790 803.475 0 462 153 488 460 3 056024Comparative Example 4795803.87504522034864603048024Comparative Example 5795803.87504522034864603052024Comparative Example 6795803.87504522034864603056024Invention Example 1805804.77504423034824603048024Invention Example 2805 804.77504423034824603052024Invention Example 3805804.77504423034824603056024Invention Example 4820805.97504324534764603048024Invention Example 5820805.97504324534764603052024Invention Example 6820805.975043245 34764603056024Invention Example 7840807.57504225534724603048024Invention Example 8840807.57504225534724603052024Invention Example 9840807.57504225534724603056024Comparative Example 7820805.9750432451764603056024

[0163] Referring to Table 2, Invention Example 1-9 satisfies all of the conditions of annealing heat treatment temperature: 800 to 850°C, annealing heat treatment holding time: 60 to 130 seconds, slow cooling rate: less than 20°C, slow cooling end temperature: 650 to 750°C, rapid cooling rate: more than 20°C, rapid cooling end temperature: 200 to 300°C, holding time after rapid cooling: 20 to 60 seconds, heating rate before reheating heat treatment: 40°C / s or more, reheating heat treatment temperature: 350 to 470°C, reheating heat treatment holding time: 20 to 50 seconds, alloying heat treatment temperature: 480 to 560°C, and alloying heat treatment holding time: 15 to 45 seconds.

[0164] In contrast, Comparative Examples 1-6 do not satisfy and fall short of the range of annealing heat treatment temperature: 800 to 850°C, and Comparative Example 7 does not satisfy and falls short of the range of holding time: 20 to 60 seconds after rapid cooling.

[0165] Table 3 shows the microstructure phase fraction (unit: %) of the base steel sheet constituting the plated steel sheet according to the first experimental example of the present invention and the austenite decomposition rate in the alloying heat treatment step. In Table 3, F in the microstructure represents the area fraction of ferrite, TM represents the area fraction of tempered martensite, B represents the area fraction of bainite, RA represents the area fraction of retained austenite, FM represents the area fraction of fresh martensite, and P represents the area fraction of pearlite. TM+B represents the sum of the area fractions of tempered martensite and bainite, and FM+P represents the sum of the area fractions of pearlite and fresh martensite. The above area fraction represents the area ratio derived from an image acquired with a scanning electron microscope as a microstructure photograph of the base steel sheet using an image analyzer. Meanwhile, γ decomposition rate @ GA represents the decomposition rate of austenite in the alloying heat treatment step and can be calculated by the following Equation 1.

[0166] (Formula 1)

[0167] (X Pearlite + X Fresh Martensite ) / (100 - X Ferrite - X Tempered Martensite - XBainite )

[0168] (above X Pearlite is the value of the area fraction of pearlite (unit: %), and the above X Fresh Martensite is the value of the area fraction of fresh martensite (unit: %), and the above X Ferrite is the value of the area fraction of ferrite (unit: %), and X Tempered Martensite is the value of the area fraction of tempered martensite (unit: %), and the above X Bainite is the value of the area fraction of bainite (unit: %)

[0169] FTMBRAFMPTM+BFM+Pγ Decomposition rate @ GAComparative Example 1 33.5 40.52.2 1 46.43.44 2.79.80.412Comparative Example 2 34.6 41.32.9 1 1.45.74.144.29.80.462Comparative Example 3 32.3 42.43.1 1 0.95.55.845.51 1.30.509Comparative Example 4 36.5 41.73.4 10.64.92.945.17.80.4 24Comparative Example 535.440.63.710.46.13.844.39.90.488Comparative Example 634.941.33.29.95.45.344.510.70.519Invention Example 137.139.52.715.83.41.542.24.90.237Invention Example 236.340.33.614.23.52.143.95.60. 283 Invention Example 335.941.23.414.73.21.644.64.80.246 Invention Example 438.343.33.311.43.10.646.63.70.245 Invention Example 537.240.53.214.22.62.343.74.90.257 Invention Example 636.441.83.512.92.72.745.35.40 .295 Invention example 737.639.93.315.52.7143.23.70.193 Invention example 838.441.83.911.52.91.545.74.40.277 Invention example 935.640.73.814.92.42.644.550.251 Comparative example 735.242.22.911.93.44.445.17.80.396

[0170] Referring to Table 3, Invention Examples 1-9 satisfy all of the ranges in which the final microstructure of the base steel sheet of the invention example is, in terms of area fraction, ferrite: 30 to 50%, retained austenite: 5 to 20%, the sum of tempered martensite and bainite: 30 to 65%, and the remainder is at least one of pearlite and fresh martensite, and satisfy all of the ranges in which the value of the above formula 1 is 0.3 or less.

[0171] In contrast, Comparative Examples 1-6 and 7 are comparative examples, and it can be confirmed that the value of the above formula 1 does not satisfy the range of 0.3 or less and exceeds it. In Comparative Examples 1-6 and 7, it can be confirmed that the degree to which austenite is decomposed into at least one of pearlite and fresh martensite in the alloying heat treatment step is higher than in the inventive examples.

[0172] Table 4 shows the properties of the galvanized steel sheet according to the first experimental example of the present invention. In Table 4, YP represents the yield strength (unit: MPa), TS represents the tensile strength (unit: MPa), U.El represents the uniform elongation (unit: %) before local elongation occurs, and T.El represents the total elongation (unit: %) until fracture.

[0173] YPTSU.ElT.ElMaterial Satisfaction Comparison Example 1684110613.220.8X Comparison Example 2672109812.719.2X Comparison Example 3689109012.318.4X Comparison Example 4675107814.120.6X Comparison Example 5666108613.420.1X Comparison Example 6671107412.618.3X Invention Example 1668103014.822.3O Invention Example 2651102714.321 .9O invention example 3657101913.921.3O invention example 4632102115.223.1O invention example 5645101314.722.6O invention example 6636101614.622.1O invention example 761199314.923.3O invention example 860798514.721.9O invention example 961398314.321.7O comparative example 7641100413.320.5X

[0174] Referring to Table 4, Invention Examples 1-9, as invention examples, satisfy all of the following: yield strength: 600 to 760 MPa, tensile strength: 980 MPa or more, and total elongation: 21% or more.

[0175] In contrast, Comparative Examples 1-6 and 7 do not satisfy the total elongation of 21% or more as comparative examples and are all below this. This is analyzed to be because in Comparative Examples 1-6 and 7, the degree to which austenite is decomposed into at least one of pearlite and fresh martensite during the alloying heat treatment step is higher than in the inventive examples.

[0176] According to the microstructure of the base steel sheet among the plated steel sheets according to the comparative examples in Table 4, the effect of the alloying component of silicon (Si) was confirmed. Specifically, it was effective in suppressing carbide formation at a low alloying temperature (480℃), but it was confirmed that the effect of suppressing carbide formation was small at a high alloying temperature (520℃ / 560℃), and a large amount of cementite (θ) phase or pearlite (P) was generated. In other words, the chemical stability within austenite is reduced due to the carbides formed during the alloying process, so the phase transformation from austenite to pearlite or fresh martensite occurs relatively frequently during the final cooling process after the alloying heat treatment, and it was confirmed that the rate of reduction of retained austenite is large depending on the alloying temperature.

[0177] According to the microstructure of the base steel sheet among the plated steel sheets according to the invention examples in Table 4, the effect of the alloying component of aluminum (Al) was confirmed. Specifically, although some carbide formation was confirmed at a low alloying temperature (480℃), it was confirmed that the effect of suppressing carbide formation was higher than that of the comparative example at a high alloying temperature (520℃ / 560℃). That is, since the carbide formed during the alloying process is small, the chemical stability within austenite is improved compared to the comparative example, and therefore the phase transformation from austenite to pearlite or fresh martensite is relatively small during the final cooling process after the alloying heat treatment, and the reduction rate of retained austenite was confirmed to be small.

[0178] Meanwhile, referring to Comparative Examples 1 to 3 and Invention Examples 7 to 9, a decrease in tensile strength as the alloying temperature increases is commonly observed, and this is analyzed to be because transformation-induced plasticity was not expressed due to the tempering effect of the matrix structure and decomposition of residual austenite as the alloying heat treatment progressed.

[0179] Meanwhile, in Comparative Examples 1 to 3, which are silicon additives, higher strength is shown under the same conditions than in Invention Examples 7 to 9, which are aluminum additives. This is understood to be because there is a difference in the solid solution strengthening effect of aluminum / silicon and transformation-induced plasticity is not expressed due to decomposition of residual austenite.

[0180] Meanwhile, in the case of Invention Examples 7 to 9, which are aluminum additives, a higher elongation is observed under the same conditions compared to Comparative Examples 1 to 3, which are silicon additives. This is analyzed to be because transformation-induced plasticity is not expressed due to decomposition of residual austenite.

[0181] Ultimately, it can be understood that suppressing the phase transformation of residual austenite during alloying heat treatment is effective in securing material (especially elongation).

[0182] Table 5 shows the interface-related properties of the plated steel sheet according to the first experimental example of the present invention. In Table 5, the Γ phase ratio (A) and the liquid Zn ratio (B) represent the volume fraction of the gamma (Γ) phase, which is an alloy phase, and the volume fraction of the liquid pure zinc (Zn) phase, respectively, in a region within 0.1 ㎛ in the direction of the base steel sheet from the interface between the base steel sheet and the plating layer immediately after spot welding for the plated steel sheet. The A / B item represents the ratio of the volume fraction of the gamma (Γ) phase, which is an alloy phase, to the volume fraction of the liquid pure zinc (Zn) phase. The POST UTS energy reduction rate represents the strain energy reduction rate for the plated steel sheet, which is a plated material, from the point where the maximum tensile strength (UTS) appears to the point where fracture occurs during the glibble evaluation, based on the base steel sheet, which is an unplated material. The volume fraction can be measured using an optical microscope, but in this experimental example, it was measured using a Cu-Kα X-ray diffraction analyzer.

[0183] Γ phase ratio (A) (vol.%) Liquid Zn ratio (B) (vol.%) A / BPOST UTS energy Reduction rate (%) Plating material satisfaction Comparison example 1 48 52 0.9284 X Comparison example 2 43 57 0.7579 X Comparison example 3 45 55 0.8283 X Comparison example 4 49 51 0.9676 X Comparison example 5 44 56 0.7985 X Comparison example 6 43 57 0.7588 X Invention example 1 69 31 2.2365 O Invention example 2 65 35 1.8660 O Invention example 3 68 32 2.1369 O Invention example 4 67 33 2.0354 O Invention example 5 7 3 27 2.7063 O Invention example 6 7 5 25 3.0066 O Invention example 7 7 3 27 2.7052 O Invention example 8 7 7 23 3.3558 O Invention example 9 8 0 20 4.0055 O

[0184] Referring to Table 5, Invention Example 1-9 is an invention example, and it can be confirmed that, immediately after spot welding on a plated steel sheet, in a region within 0.1㎛ in the direction of the plated steel sheet from the interface between the base steel sheet and the plated layer, the volume fraction of the gamma (Γ) phase, which is an alloy phase, is 50% or more, and the ratio of the volume fraction of the gamma (Γ) phase, which is an alloy phase, to the volume fraction of the liquid pure zinc (Zn) phase is 1.5 or more. In addition, when the glibble evaluation is performed, it can be confirmed that the strain energy from the point where the maximum tensile strength (UTS) appears to the point where the fracture occurs is an energy reduction rate of 70% or less for the plated steel sheet, which is a plated material, based on the base steel sheet, which is an unplated material. That is, it can be confirmed that the strain energy from the point where the stress is the highest in a tensile test on the plated steel sheet to the point of fracture is 30% or more of the strain energy from the point where the stress is the highest in a tensile test on the base steel sheet to the point of fracture.

[0185] In the invention example, it was confirmed that the liquid metal embrittlement (LME) characteristics of QP steel or TRIP steel plating materials can be improved through component control and process condition control. More specifically, by controlling the total amount and ratio of silicon (Si) and aluminum (Al) alloys to configure the component system, and by performing an appropriate heat treatment process to manufacture a QP / TRIP plating steel sheet of the target material, and then performing spot welding, it can be confirmed that the amount of aluminum (Al) diffusion to the interface between the plating layer and the steel sheet changes depending on the total amount and ratio of silicon (Si) and aluminum (Al) alloys, and accordingly, liquid metal embrittlement (LME) resistance can be secured due to changes in phase stability in the interface region.

[0186] That is, the stability of the Γ phase increases due to changes in the interface composition and phase stability caused by Al interfacial diffusion, and the fraction of the interface liquid Zn decreases due to a decrease in the stability of the liquid Zn, so that the amount of penetration into the base steel sheet is reduced due to the reduction in the interface liquid Zn, thereby ensuring liquid metal embrittlement (LME) resistance.

[0187] In contrast, Comparative Example 1-6 is a comparative example in which, immediately after spot welding, the volume fraction of the gamma (Γ) phase, which is an alloy phase, in the region within 0.1 ㎛ in the direction of the base steel sheet from the interface between the base steel sheet and the plating layer is below 50% and does not satisfy the range of 1.5 or more and falls below it, and when the glib evaluation is performed, the strain energy from the point where the maximum tensile strength (UTS) appears to the point where the fracture occurs is confirmed to exceed the range of energy reduction rates of 70% or less for the plated steel sheet, which is a plated material, based on the base steel sheet, which is an unplated material.

[0188] According to the comparative example, the stability of the Γ phase decreases due to changes in the interface composition and phase stability caused by Al interfacial diffusion, and the interface liquid Zn fraction increases due to increased stability of the liquid Zn, so it can be analyzed that the penetration amount into the base steel sheet increases due to the increase in the interface liquid Zn, thereby deteriorating the liquid metal embrittlement (LME) resistance.

[0189] Experimental example 2 (1.2 GPa-grade galvanized steel plate)

[0190] Table 6 shows the composition (unit: weight%) of the main components of the base steel sheet constituting the plated steel sheet according to the second experimental example of the present invention. In addition, the composition of phosphorus (P): 0.01%, sulfur (S): 0.003%, nitrogen (N): 0.003%, and boron (B): 0.002% was applied in the same manner.

[0191] CSiMnAl[Si] / [Al] <4 Satisfaction Comparison Example 10.21.54-X Comparison Example 20.21.54-X Comparison Example 30.21.54-X Comparison Example 40.21.2540.25X Comparison Example 50.21.2540.25X Comparison Example 60.21.2540.25X Invention Example 10.2140.51O Invention Example 20.2140.51O Invention Example 40.20.541O Invention Example 50.20.541O Invention Example 60.20.541O Comparison Example 70.20.541O Invention Example 70.2-41.5O Invention Example 80.2-41.5O Invention Example 90.2-41.5O

[0192] Referring to Table 6, Invention Example 1-9 contains, in wt%, carbon (C): 0.1 to 0.3% (strictly, 0.1 to 0.25%), silicon (Si): 0 to 2.0%, manganese (Mn): 2.0 to 4.0% (strictly, 2.5 to 4.0%), aluminum (Al): more than 0 to 2.0%, phosphorus (P): more than 0 to 0.02% or less, sulfur (S): more than 0 to 0.005% or less, nitrogen (N): more than 0 to 0.006% or less, boron (B): more than 0 to 0.003% or less, and satisfies all the composition ranges of iron (Fe), and the sum of the content of silicon (Si) and the content of aluminum (Al) is 2.0% or less, and the ratio of the content of silicon (Si) and the content of aluminum (Al) is less than 4.0. Everyone is satisfied.

[0193] Meanwhile, Invention Example 7-9 is a case where silicon (Si) was not intentionally added. However, in this case, it can be understood that silicon (Si) is contained at 0.01% or less. This is because even if silicon (Si) is not intentionally added, a very small amount can be detected as an inevitable impurity.

[0194] In contrast, Comparative Example 1-6 does not satisfy the condition that the ratio of the content of silicon (Si) to the content of aluminum (Al) is less than 4.0.

[0195] Table 7 shows the process conditions for manufacturing a plated steel sheet according to the second experimental example of the present invention. The unit of the temperature item is ℃, the unit of the speed item is ℃ / s, and the units of the time and maintenance items are seconds (s). In the experimental example of the present invention, other process conditions were applied as conditions that satisfied the process conditions described above but had the same single value. For example, the hot rolling process applied the process conditions of reheating temperature: 1200℃, finishing rolling temperature: 900℃, and coiling temperature: 600℃.

[0196] Annealing temperature, Annealing time, Slow cooling rate, Slow cooling temperature, Rapid cooling rate, Rapid cooling temperature, Rapid cooling holding, Reheating rate, Reheating temperature, Reheating holding, Alloying temperature, Alloying holding, Comparative example 1 800 803.475 0 462 303 488 460 304 802 4 Comparative example 2 800 803.475 0 462 303 488 460 305 202 4 Comparative example 3 800 803.475 0 462 303 488 460 305 202 4 034884603056024Comparative Example 4805803.87504523534864603048024Comparative Example 5805803.87504523534864603052024Comparative Example 6805803.87504523534864603056024Invention Example 1815804.7750442453482 4603048024 Invention Example 2815804.77504424534824603052024 Invention Example 4830805.97504326034764603048024 Invention Example 5830805.97504326034764603052024 Invention Example 6830805.97504326034764603 056024Comparative Example 7830805.9750432601764603056024Invention Example 7850807.57504227034724603048024Invention Example 8850807.57504227034724603052024Invention Example 9850807.57504227034724603056024

[0197] Referring to Table 7, Invention Example 1-9 satisfies all of the conditions of annealing heat treatment temperature: 800 to 850°C, annealing heat treatment holding time: 60 to 130 seconds, slow cooling rate: less than 20°C, slow cooling end temperature: 650 to 750°C, rapid cooling rate: more than 20°C, rapid cooling end temperature: 200 to 300°C, holding time after rapid cooling: 20 to 60 seconds, heating rate before reheating heat treatment: 40°C / s or more, reheating heat treatment temperature: 350 to 470°C, reheating heat treatment holding time: 20 to 50 seconds, alloying heat treatment temperature: 480 to 560°C, and alloying heat treatment holding time: 15 to 45 seconds.

[0198] In contrast, Comparative Example 7 does not satisfy the range of 20 to 60 seconds of holding time after rapid cooling and falls below this range.

[0199] Table 8 shows the microstructure phase fraction (unit: %) of the base steel sheet constituting the plated steel sheet according to the second experimental example of the present invention and the austenite decomposition rate in the alloying heat treatment step. In Table 8, the microstructure F represents the area fraction of ferrite, TM represents the area fraction of tempered martensite, B represents the area fraction of bainite, RA represents the area fraction of retained austenite, FM represents the area fraction of fresh martensite, and P represents the area fraction of pearlite. TM+B represents the sum of the area fractions of tempered martensite and bainite, and FM+P represents the sum of the area fractions of pearlite and fresh martensite. Meanwhile, the γ decomposition rate @ GA represents the decomposition rate of austenite in the alloying heat treatment step, and can be calculated by the following equation 1.

[0200] (Formula 1)

[0201] (X Pearlite + X Fresh Martensite ) / (100 - X Ferrite - X Tempered Martensite - X Bainite )

[0202] (above X Pearlite is the value of the area fraction of pearlite (unit: %), and the above X Fresh Martensite is the value of the area fraction of fresh martensite (unit: %), and the above XFerrite is the value of the area fraction of ferrite (unit: %), and X Tempered Martensite is the value of the area fraction of tempered martensite (unit: %), and the above X Bainite is the value of the area fraction of bainite (unit: %)

[0203] FTMBRAFMPTM+BFM+Pγ Decomposition rate @ GAComparative example 1 10.15 1.2 10.9 18.36.8 2.7 6 2.19.5 0.34 2Comparative example 2 11.34 8.5 15.6 14.44.8 5.46 4.110.20.415Comparative example 3 10.65 0.3 12.2 12.8 7 7.16 2.5 14.10.5 24Comparative example 4 13.24 8.4 13.4 17.44.82. 861.87.60.304Comparative Example 512.650.211.815.74.84.9629.70.382Comparative Example 614.149.513.111.16.35.962.612.20.524Invention Example 111.352.311.319.34.61.263.65.80.231Invention Example 211.550.712. 617.74.82.763.37.50.298 Invention Example 412.950.613.418.73.50.9644.40.190 Invention Example 512.351.310.618.64.13.161.97.20.279 Invention Example 612.152.511.417.82.53.763.96.20.258 Comparative Example 711 .351.410.216.44.95.861.610.70.395Invention Example 712.751.614.817.33.6066.43.60.172Invention Example 811.65114.318.43.31.465.34.70.203Invention Example 912.851.513.617.222.965.14.90.222

[0204] Referring to Table 8, Invention Examples 1-9 satisfy all of the ranges in which the final microstructure of the base steel sheet of the invention example is, in terms of area fraction, ferrite: 5 to 25%, retained austenite: 5 to 20%, the sum of tempered martensite and bainite: 30 to 90%, and the remainder is at least one of pearlite and fresh martensite, and satisfy all of the ranges in which the value of the above formula 1 is 0.3 or less.

[0205] In contrast, Comparative Example 1-7 is a comparative example, and it can be confirmed that the value of the above formula 1 does not satisfy the range of 0.3 or less and exceeds it. That is, it can be confirmed that in Comparative Example 1-7, the degree to which austenite is decomposed into at least one of pearlite and fresh martensite in the alloying heat treatment step is higher than in the inventive example.

[0206] Table 9 shows the properties of the plated steel sheet according to the second experimental example of the present invention. In Table 9, YP represents the yield strength (unit: MPa), TS represents the tensile strength (unit: MPa), U.El represents the uniform elongation (unit: %) before local elongation occurs, and T.El represents the total elongation (unit: %) until fracture.

[0207] YPTSU.ElT.El Material Satisfaction Comparison Example 197113449.211.8X Comparison Example 296613097.59.6X Comparison Example 399812586.99.1X Comparison Example 498313219.212.9X Comparison Example 595113167.510.8X Comparison Example 695512886.99.5X Invention Example 196712237.414.5O Invention Example 2934122 58.314.2O Invention Example 493512167.714.5O Invention Example 589912148.215.1O Invention Example 687411978.714.2O Comparative Example 793213077.611.2X Invention Example 792512118.315.9O Invention Example 889512018.815.3O Invention Example 985111849.114.7O

[0208] Referring to Table 9, Invention Example 1-9 satisfies all of the following: yield strength: 850 to 1070 MPa, tensile strength: 1180 MPa or more, and total elongation: 14% or more.

[0209] In contrast, it can be confirmed that Comparative Example 1-7 does not satisfy the total elongation of 14% or more and falls below it all. This is analyzed to be because in Comparative Example 1-7, the degree to which austenite is decomposed into at least one of pearlite and fresh martensite during the alloying heat treatment step is higher than in the inventive example.

[0210] Referring to the photographs of the microstructure of the base steel sheets among the plated steel sheets according to Comparative Examples 1 to 3, the effect of the alloying component of silicon (Si) can be confirmed. Specifically, it is effective in suppressing carbide formation at a low alloying temperature (480℃), but it is less effective in suppressing carbide formation at a high alloying temperature (520℃ / 560℃), and it can be confirmed that a large amount of cementite (θ) phase or pearlite (P) is generated. In other words, the chemical stability of austenite is reduced due to the carbides formed during the alloying process, so that a relatively large number of phase transformations from austenite to pearlite or fresh martensite occur during the final cooling process after the alloying heat treatment, and it can be confirmed that the rate of reduction of retained austenite is large depending on the alloying temperature.

[0211] Referring to the photographs of the microstructure of the base steel sheet among the plated steel sheets according to Invention Examples 7 to 9, the effect of the alloying component of aluminum (Al) can be confirmed. Specifically, although some carbide formation is confirmed at a low alloying temperature (480°C), it can be confirmed that the effect of suppressing carbide formation at a high alloying temperature (520°C / 560°C) is higher than in Comparative Examples 1 to 3. That is, since the carbide formed during the alloying process is small, the chemical stability within austenite is improved compared to Comparative Examples 1 to 3, and therefore the phase transformation from austenite to pearlite or fresh martensite during the final cooling process after the alloying heat treatment is relatively small, and it can be confirmed that the rate of reduction of retained austenite is small.

[0212] Meanwhile, referring to Comparative Examples 1 to 3 and Invention Examples 7 to 9, a decrease in tensile strength as the alloying temperature increases is commonly observed, and this is analyzed to be because transformation-induced plasticity was not expressed due to the tempering effect of the matrix structure and decomposition of residual austenite as the alloying heat treatment progressed.

[0213] Meanwhile, in Comparative Examples 1 to 3, which are silicon additives, higher strength is shown under the same conditions than in Invention Examples 7 to 9, which are aluminum additives. This is understood to be because there is a difference in the solid solution strengthening effect of aluminum / silicon and transformation-induced plasticity is not expressed due to decomposition of residual austenite.

[0214] Meanwhile, in the case of Invention Examples 7 to 9, which are aluminum additives, a higher elongation is observed under the same conditions compared to Comparative Examples 1 to 3, which are silicon additives. This is analyzed to be because transformation-induced plasticity is not expressed due to decomposition of residual austenite.

[0215] Ultimately, it can be understood that suppressing the phase transformation of residual austenite during alloying heat treatment is effective in securing material (especially elongation).

[0216] Table 10 shows the interface-related properties of the plated steel sheet according to the second experimental example of the present invention. In Table 10, the Γ phase ratio (A) and the liquid Zn ratio (B) represent the volume fraction of the gamma (Γ) phase, which is an alloy phase, and the volume fraction of the liquid pure zinc (Zn) phase, respectively, in a region within 0.1 ㎛ in the direction of the base steel sheet from the interface between the base steel sheet and the plating layer immediately after spot welding for the plated steel sheet, and the A / B item represents the ratio of the volume fraction of the gamma (Γ) phase, which is an alloy phase, to the volume fraction of the liquid pure zinc (Zn) phase, and the POST UTS energy reduction rate represents the strain energy reduction rate for the plated steel sheet, which is a plated material, from the point where the maximum tensile strength (UTS) appears to the point where fracture occurs during the glibble evaluation, based on the base steel sheet, which is an unplated material.

[0217] Γ phase ratio (A) (vol.%) Liquid Zn ratio (B) (vol.%) A / BPOST UTS energy Reduction rate (%) Plating material satisfaction Comparison example 1 44560.7981 X Comparison example 2 46540.8583 X Comparison example 3 43570.7577 X Comparison example 4 45550.8282 X Comparison example 5 47530.8980 X Comparison example 6 41590.6984 X Invention example 166341.9466 O Invention example 263371.7061 O Invention example 4 73272.7057 O Invention example 5 76243.1759 O Invention example 6 75253.0053 O Invention example 7 83174.8851 O Invention example 8 82184.5649 O Invention example 9 85155.6755 O

[0218] Referring to Table 10, in Invention Example 1-9, it can be confirmed that, immediately after spot welding on the plated steel sheet, in a region within 0.1㎛ in the direction of the plated steel sheet from the interface between the base steel sheet and the plated layer, the volume fraction of the gamma (Γ) phase, which is an alloy phase, is 50% or more, and the ratio of the volume fraction of the gamma (Γ) phase, which is an alloy phase, to the volume fraction of the liquid pure zinc (Zn) phase is 1.5 or more. In addition, when the glibble evaluation is performed, it can be confirmed that the strain energy from the point where the maximum tensile strength (UTS) appears to the point where the fracture occurs is an energy reduction rate of 70% or less for the plated steel sheet, which is a plated material, based on the base steel sheet, which is an unplated material. That is, it can be confirmed that the strain energy from the point where the stress is the highest to the point of fracture in the tensile test on the plated steel sheet is 30% or more of the strain energy from the point where the stress is the highest to the point of fracture in the tensile test on the base steel sheet.

[0219] In the invention example, it was confirmed that the liquid metal embrittlement (LME) characteristics of QP steel or TRIP steel plating materials can be improved through component control and process condition control. More specifically, by controlling the total amount and ratio of silicon (Si) and aluminum (Al) alloys to configure the component system, and by performing an appropriate heat treatment process to manufacture a QP / TRIP plating steel sheet of the target material, and then performing spot welding, it can be confirmed that the amount of aluminum (Al) diffusion to the interface between the plating layer and the steel sheet changes depending on the total amount and ratio of silicon (Si) and aluminum (Al) alloys, and accordingly, liquid metal embrittlement (LME) resistance can be secured due to changes in phase stability in the interface region.

[0220] That is, the stability of the Γ phase increases due to changes in the interface composition and phase stability caused by Al interfacial diffusion, and the fraction of the interface liquid Zn decreases due to a decrease in the stability of the liquid Zn, so that the amount of penetration into the base steel sheet is reduced due to the reduction in the interface liquid Zn, thereby ensuring liquid metal embrittlement (LME) resistance.

[0221] In contrast, Comparative Example 1-6 is a comparative example in which, immediately after spot welding, the volume fraction of the gamma (Γ) phase, which is an alloy phase, in the region within 0.1 ㎛ in the direction of the base steel sheet from the interface between the base steel sheet and the plating layer is below 50% and does not satisfy the range of 1.5 or more and falls below it, and when the glib evaluation is performed, the strain energy from the point where the maximum tensile strength (UTS) appears to the point where the fracture occurs is confirmed to exceed the range of energy reduction rates of 70% or less for the plated steel sheet, which is a plated material, based on the base steel sheet, which is an unplated material.

[0222] According to the comparative example, the stability of the Γ phase decreases due to changes in the interface composition and phase stability caused by Al interfacial diffusion, and the interface liquid Zn fraction increases due to increased stability of the liquid Zn, so it can be analyzed that the penetration amount into the base steel sheet increases due to the increase in the interface liquid Zn, thereby deteriorating the liquid metal embrittlement (LME) resistance.

[0223] Experimental example 3 (1.5 GPa-grade galvanized steel plate)

[0224] Table 11 shows the composition (unit: weight%) of the main components of the base steel sheet constituting the plated steel sheet according to the third experimental example of the present invention. In addition, the composition of phosphorus (P): 0.01%, sulfur (S): 0.003%, nitrogen (N): 0.003%, and boron (B): 0.002% was applied in the same manner.

[0225] CSiMnAl[Si] / [Al] <4 Satisfaction Comparison Example 10.35 1.52-X Comparison Example 20.35 1.52-X Comparison Example 30.35 1.52-X Comparison Example 40.35 1.25 20.25X Comparison Example 50.35 1.25 20.25X Comparison Example 60.35 1.25 20.25X Invention Example 10.35 120.51O Invention Example 20.35 120.51O Invention Example 30.35 120.51O Invention Example 40.35 0.521O Invention Example 50.35 0.521O Invention Example 60.35 0.521O Invention Example 70.35-21.5O Invention Example 80.35-21.5O Invention Example 90.35-21.5O Comparison Example 70.35 0.521O

[0226] Referring to Table 11, Invention Example 1-9 contains, in wt%, carbon (C): 0.2 to 0.4% (strictly, more than 0.25% to 0.4% or less), silicon (Si): 0 to 2.0%, manganese (Mn): 1.5 to 4.0% (strictly, more than 2.0% to 4.0% or less), aluminum (Al): more than 0 to 2.0% or less, phosphorus (P): more than 0 to 0.02% or less, sulfur (S): more than 0 to 0.005% or less, nitrogen (N): more than 0 to 0.006% or less, boron (B): more than 0 to 0.003% or less, and the remainder is iron (Fe), satisfying all composition ranges, and the sum of the content of silicon (Si) and the content of aluminum (Al) is 2.0% or less, and the ratio of the content of silicon (Si) to the content of aluminum (Al) is 4.0 All conditions below are satisfied.

[0227] Meanwhile, Invention Example 7-9 is a case where silicon (Si) was not intentionally added. However, in this case, it can be understood that silicon (Si) is contained at 0.01% or less. This is because even if silicon (Si) is not intentionally added, a very small amount can be detected as an inevitable impurity.

[0228] In contrast, Comparative Example 1-6 does not satisfy the condition that the ratio of the content of silicon (Si) to the content of aluminum (Al) is less than 4.0.

[0229] Table 12 shows the process conditions for manufacturing a plated steel sheet according to the third experimental example of the present invention. The unit of the temperature item is ℃, the unit of the speed item is ℃ / s, and the unit of the time and maintenance items is second (s). In the experimental example of the present invention, other process conditions were applied as conditions that satisfied the process conditions described above but had the same single value. For example, the hot rolling process applied the process conditions of reheating temperature: 1200℃, finishing rolling temperature: 900℃, and coiling temperature: 600℃.

[0230] Annealing temperature, Annealing time, Slow cooling rate, Slow cooling temperature, Rapid cooling rate, Rapid cooling temperature, Rapid cooling holding, Reheating rate, Reheating temperature, Reheating holding, Alloying temperature, Alloying holding, Comparative example 1 835 803.475 0 46 20 5 3 488 46 0 3 0 480 24 Comparative example 2 835 803.475 0 46 20 5 3 488 46 0 3 0 5 20 24 Comparative example 3 835 803.475 0 46 20 5 3 488 46 0 3 056024Comparative Example 4840803.87504521034864603048024Comparative Example 5840803.87504521034864603052024Comparative Example 6840803.87504521034864603056024Invention Example 1850804.77504422034824603048024Invention Example 2850 804.77504422034824603052024Invention Example 3850804.77504422034824603056024Invention Example 4865805.97504323534764603048024Invention Example 5865805.97504323534764603052024Invention Example 6865805.975043235 34764603056024Invention Example 7885807.57504224534724603048024Invention Example 8885807.57504224534724603052024Invention Example 9885807.57504224534724603056024Comparative Example 7865805.9750432351764603056024

[0231] Referring to Table 12, Invention Example 1-9 satisfies all of the conditions of annealing heat treatment temperature: 850 to 900°C, annealing heat treatment holding time: 60 to 130 seconds, slow cooling rate: less than 20°C, slow cooling end temperature: 650 to 750°C, rapid cooling rate: more than 20°C, rapid cooling end temperature: 200 to 300°C, holding time after rapid cooling: 20 to 60 seconds, heating rate before reheating heat treatment: 40°C / s or more, reheating heat treatment temperature: 350 to 470°C, reheating heat treatment holding time: 20 to 50 seconds, alloying heat treatment temperature: 480 to 560°C, and alloying heat treatment holding time: 15 to 45 seconds.

[0232] In contrast, Comparative Examples 1-6 do not satisfy and fall short of the range of annealing heat treatment temperature: 850 to 900°C, and Comparative Example 7 does not satisfy and falls short of the range of holding time: 20 to 60 seconds after rapid cooling.

[0233] Table 13 shows the microstructure phase fractions (unit: %) of the base steel sheet constituting the plated steel sheet according to the third experimental example of the present invention and the austenite decomposition rate in the alloying heat treatment step. In Table 13, the microstructure F represents the area fraction of ferrite, TM represents the area fraction of tempered martensite, B represents the area fraction of bainite, RA represents the area fraction of retained austenite, FM represents the area fraction of fresh martensite, and P represents the area fraction of pearlite. TM+B represents the sum of the area fractions of tempered martensite and bainite, and FM+P represents the sum of the area fractions of pearlite and fresh martensite. Meanwhile, the γ decomposition rate @ GA represents the decomposition rate of austenite in the alloying heat treatment step, and can be calculated by the following equation 1.

[0234] (Formula 1)

[0235] (X Pearlite + X Fresh Martensite ) / (100 - X Ferrite - X Tempered Martensite - X Bainite )

[0236] (above X Pearliteis the value of the area fraction of pearlite (unit: %), and the above X Fresh Martensite is the value of the area fraction of fresh martensite (unit: %), and the above X Ferrite is the value of the area fraction of ferrite (unit: %), and X Tempered Martensite is the value of the area fraction of tempered martensite (unit: %), and the above X Bainite is the value of the area fraction of bainite (unit: %)

[0237] FTMBRAFMPTM+BFM+Pγ Decomposition rate @ GAComparative example 12.36 9.83.4 14.77.12.79.80 73.20 0.40Comparative example 23.57 0.33.11 3.65.14.49.50 73.40 0.41Comparative example 32.27 1.23.5 106.86.3 13.107 4.70 0.57Comparative example 41.37 2.44.114.15 3.18.1076. 500.36Comparative Example 5273.33.811.64.74.69.3077.100.44Comparative Example 61.572.55.19.26.25.511.7077.600.56Invention Example 1173.83.1164.91.26.1076.900.28Invention Example 20.574.23.515.34.32.26.5077 .700.30Invention Example 30.872.13.716.43.73.37.0075.800.30Invention Example 4075.53.616.73.50.74.2079.100.20Invention Example 5074.23.816.23.32.55.8078.000.26Invention Example 6075.63.1152.63.76.3078. 700.30 Invention example 7075.43.5173.20.94.1078.900.19 Invention example 8074.63.417.42.91.74.6078.000.21 Invention example 9073.33.2182.23.35.5076.500.23 Comparative example 7076.12.812.44.34.48.7078.900.41

[0238] Referring to Table 13, Invention Example 1-9 satisfies all of the ranges in which the final microstructure of the base steel sheet of the invention example is, in terms of area fraction, ferrite: 0 to 5%, retained austenite: 5 to 35%, the sum of tempered martensite and bainite: 30 to 95%, and the remainder is at least one of pearlite and fresh martensite, and satisfies all of the ranges in which the value of the above formula 1 is 0.3 or less.

[0239] In contrast, Comparative Examples 1-6 and 7 are comparative examples, and it can be confirmed that the value of the above formula 1 does not satisfy the range of 0.3 or less and exceeds it. That is, it can be confirmed that in Comparative Examples 1-6 and 7, the degree to which austenite is decomposed into at least one of pearlite and fresh martensite in the alloying heat treatment step is higher than in the inventive examples.

[0240] Table 14 shows the properties of the plated steel sheet according to the third experimental example of the present invention. In Table 14, YP represents the yield strength (unit: MPa), TS represents the tensile strength (unit: MPa), U.El represents the uniform elongation (unit: %) before local elongation occurs, and T.El represents the total elongation (unit: %) until fracture.

[0241] YPTSU.ElT.El Material Satisfaction Comparison Example 1 1 1 8 3 1 5 6 8 5.8 1 1.5 X Comparison Example 2 1 1 7 8 1 5 7 6.4 1 0.6 X Comparison Example 3 1 2 1 0 1 5 5 9 6.6 9.7 X Comparison Example 4 1 1 6 5 1 5 5 7 6.7 1 1.3 X Comparison Example 5 1 1 5 4 1 5 6 3 7.1 1 0.5 X Comparison Example 6 1 1 7 3 1 5 6 4 6.5 8.9 X Invention Example 1 1 1 4 7 1 5 3 4 7 1 5.1 O Invention Example 2 1 1 3 9 1 5 2 1 7.2 1 4.1 O Bal Honor 3114415287.114.4O Invention Example 4113015437.114.7O Invention Example 5111015256.914.3O Invention Example 6111515147.515.2O Invention Example 7113715187.815.3O Invention Example 8110715037.915.7O Invention Example 9106314997.415.5O Comparative Example 7114315277.512.3X

[0242] Referring to Table 14, Invention Example 1-9, as an invention example, satisfies all of the following: yield strength: 1000 to 1350 MPa, tensile strength: 1480 MPa or more, and total elongation: 14% or more.

[0243] In contrast, Comparative Examples 1-6 and 7 do not satisfy the total elongation of 14% or more as comparative examples and are all below this. This is analyzed to be because in Comparative Examples 1-6 and 7, the degree to which austenite is decomposed into at least one of pearlite and fresh martensite during the alloying heat treatment step is higher than in the inventive examples.

[0244] Referring to the photographs of the microstructure of the base steel sheets among the plated steel sheets according to Comparative Examples 1 to 3, the effect of the alloying component of silicon (Si) can be confirmed. Specifically, it is effective in suppressing carbide formation at a low alloying temperature (480℃), but it is less effective in suppressing carbide formation at a high alloying temperature (520℃ / 560℃), and it can be confirmed that a large amount of cementite (θ) phase or pearlite (P) is generated. In other words, the chemical stability of austenite is reduced due to the carbides formed during the alloying process, so that a relatively large number of phase transformations from austenite to pearlite or fresh martensite occur during the final cooling process after the alloying heat treatment, and it can be confirmed that the rate of reduction of retained austenite is large depending on the alloying temperature.

[0245] Referring to the photographs of the microstructure of the base steel sheet among the plated steel sheets according to Invention Examples 7 to 9, the effect of the alloying component of aluminum (Al) can be confirmed. Specifically, although some carbide formation is confirmed at a low alloying temperature (480°C), it can be confirmed that the effect of suppressing carbide formation at a high alloying temperature (520°C / 560°C) is higher than in Comparative Examples 1 to 3. That is, since the carbide formed during the alloying process is small, the chemical stability within austenite is improved compared to Comparative Examples 1 to 3, and therefore the phase transformation from austenite to pearlite or fresh martensite during the final cooling process after the alloying heat treatment is relatively small, and it can be confirmed that the rate of reduction of retained austenite is small.

[0246] Meanwhile, referring to Comparative Examples 1 to 3 and Invention Examples 7 to 9, a decrease in tensile strength as the alloying temperature increases is commonly observed, and this is analyzed to be because transformation-induced plasticity was not expressed due to the tempering effect of the matrix structure and decomposition of residual austenite as the alloying heat treatment progressed.

[0247] Meanwhile, in Comparative Examples 1 to 3, which are silicon additives, higher strength is shown under the same conditions than in Invention Examples 7 to 9, which are aluminum additives. This is understood to be because there is a difference in the solid solution strengthening effect of aluminum / silicon and transformation-induced plasticity is not expressed due to decomposition of residual austenite.

[0248] Meanwhile, in the case of Invention Examples 7 to 9, which are aluminum additives, a higher elongation is observed under the same conditions compared to Comparative Examples 1 to 3, which are silicon additives. This is analyzed to be because transformation-induced plasticity is not expressed due to decomposition of residual austenite.

[0249] Ultimately, it can be understood that suppressing the phase transformation of residual austenite during alloying heat treatment is effective in securing material (especially elongation).

[0250] Table 15 shows the interface-related properties of the plated steel sheet according to the third experimental example of the present invention. In Table 15, the Γ phase ratio (A) and the liquid Zn ratio (B) represent the volume fraction of the gamma (Γ) phase, which is an alloy phase, and the volume fraction of the liquid pure zinc (Zn) phase, respectively, in a region within 0.1 ㎛ in the direction of the base steel sheet from the interface between the base steel sheet and the plating layer immediately after spot welding for the plated steel sheet, and the A / B item represents the ratio of the volume fraction of the gamma (Γ) phase, which is an alloy phase, to the volume fraction of the liquid pure zinc (Zn) phase, and the POST UTS energy reduction rate represents the strain energy reduction rate for the plated steel sheet, which is a plated material, from the point where the maximum tensile strength (UTS) appears to the point where fracture occurs during the glibble evaluation, based on the base steel sheet, which is an unplated material.

[0251] Γ phase ratio (A) (vol.%) Liquid Zn ratio (B) (vol.%) A / BPOST UTS energy Reduction rate (%) Plating material satisfaction Comparison example 141590.6978X Comparison example 245550.8285X Comparison example 347530.8982X Comparison example 444560.7983X Comparison example 542580.7286X Comparison example 648520.9282X Invention example 168322.1367O Invention example 265351.8662O Invention example 366341.9461O Invention example 471292.4563O Invention example 572282.5758O Invention example 674262.8554O Invention example 779213.7655O Invention example 881194.2658O Invention example 980204.0053O

[0252] Referring to Table 15, Invention Example 1-9 is an invention example, and it can be confirmed that, immediately after spot welding on a plated steel sheet, in a region within 0.1㎛ in the direction of the plated steel sheet from the interface between the base steel sheet and the plated layer, the volume fraction of the gamma (Γ) phase, which is an alloy phase, is 50% or more, and the ratio of the volume fraction of the gamma (Γ) phase, which is an alloy phase, to the volume fraction of the liquid pure zinc (Zn) phase is 1.5 or more. In addition, when the glibble evaluation is performed, it can be confirmed that the strain energy from the point where the maximum tensile strength (UTS) appears to the point where the fracture occurs is an energy reduction rate of 70% or less for the plated steel sheet, which is a plated material, based on the base steel sheet, which is an unplated material. That is, it can be confirmed that the strain energy from the point where the stress is the highest in a tensile test on the plated steel sheet to the point of fracture is 30% or more of the strain energy from the point where the stress is the highest in a tensile test on the base steel sheet to the point of fracture.

[0253] In the invention example, it was confirmed that the liquid metal embrittlement (LME) characteristics of QP steel or TRIP steel plating materials can be improved through component control and process condition control. More specifically, by controlling the total amount and ratio of silicon (Si) and aluminum (Al) alloys to configure the component system, and by performing an appropriate heat treatment process to manufacture a QP / TRIP plating steel sheet of the target material, and then performing spot welding, it can be confirmed that the amount of aluminum (Al) diffusion to the interface between the plating layer and the steel sheet changes depending on the total amount and ratio of silicon (Si) and aluminum (Al) alloys, and accordingly, liquid metal embrittlement (LME) resistance can be secured due to changes in phase stability in the interface region.

[0254] That is, the stability of the Γ phase increases due to changes in the interface composition and phase stability caused by Al interfacial diffusion, and the fraction of the interface liquid Zn decreases due to a decrease in the stability of the liquid Zn, so that the amount of penetration into the base steel sheet is reduced due to the reduction in the interface liquid Zn, thereby ensuring liquid metal embrittlement (LME) resistance.

[0255] In contrast, Comparative Example 1-6 is a comparative example in which, immediately after spot welding, the volume fraction of the gamma (Γ) phase, which is an alloy phase, in the region within 0.1 ㎛ in the direction of the base steel sheet from the interface between the base steel sheet and the plating layer is below 50% and does not satisfy the range of 1.5 or more and falls below it, and when the glib evaluation is performed, the strain energy from the point where the maximum tensile strength (UTS) appears to the point where the fracture occurs is confirmed to exceed the range of energy reduction rates of 70% or less for the plated steel sheet, which is a plated material, based on the base steel sheet, which is an unplated material.

[0256] According to the comparative example, the stability of the Γ phase decreases due to changes in the interface composition and phase stability caused by Al interfacial diffusion, and the interface liquid Zn fraction increases due to increased stability of the liquid Zn, so it can be analyzed that the penetration amount into the base steel sheet increases due to the increase in the interface liquid Zn, thereby deteriorating the liquid metal embrittlement (LME) resistance.

[0257] While the above description focuses on specific embodiments of the present invention, those skilled in the art will appreciate that various modifications and variations can be made. As long as such modifications and variations do not depart from the scope of the present invention, they are considered to be within the scope of the present invention. Therefore, the scope of the present invention should be determined by the claims set forth below.

Claims

1. A steel sheet containing, by weight%, carbon (C): 0.1 to 0.4%, silicon (Si): more than 0% and not more than 2.0%, manganese (Mn): 1.5 to 4.0%, aluminum (Al): more than 0 and not more than 2.0%, phosphorus (P): more than 0 and not more than 0.02%, sulfur (S): more than 0 and not more than 0.005%, nitrogen (N): more than 0 and not more than 0.006%, boron (B): more than 0 and not more than 0.003%, and the remainder including iron (Fe) and other unavoidable impurities; and A plated steel sheet including a plated layer on the above-mentioned steel sheet, In the above steel plate, the sum of the content of silicon (Si) and the content of aluminum (Al) is 2.0% or less, and the ratio of the content of silicon (Si) and the content of aluminum (Al) is less than 4.

0. Galvanized steel sheet.

2. In paragraph 1, The above steel sheet contains, in wt%, carbon (C): 0.1 to 0.3%, silicon (Si): more than 0% and 2.0% or less, manganese (Mn): 1.5 to 3.0%, aluminum (Al): more than 0 and 2.0% or less, phosphorus (P): more than 0 and 0.02% or less, sulfur (S): more than 0 and 0.005% or less, nitrogen (N): more than 0 and 0.006% or less, boron (B): more than 0 and 0.003% or less, and the remainder includes iron (Fe) and other unavoidable impurities. It is characterized by a yield strength of 600 to 760 MPa, a tensile strength of 980 MPa or more, and a total elongation of 21% or more. Galvanized steel sheet.

3. In paragraph 2, The final microstructure of the above steel plate is characterized by having an area fraction of ferrite: 30 to 50%, retained austenite: 5 to 20%, a total of tempered martensite and bainite: 30 to 65%, and the remainder being at least one of pearlite and fresh martensite. Galvanized steel sheet.

4. In paragraph 1, The above steel sheet contains, in wt%, carbon (C): 0.1 to 0.3%, silicon (Si): more than 0% and 2.0% or less, manganese (Mn): 2.0 to 4.0%, aluminum (Al): more than 0 and 2.0% or less, phosphorus (P): more than 0 and 0.02% or less, sulfur (S): more than 0 and 0.005% or less, nitrogen (N): more than 0 and 0.006% or less, boron (B): more than 0 and 0.003% or less, and the remainder includes iron (Fe) and other unavoidable impurities. It is characterized by a yield strength of 850 to 1070 MPa, a tensile strength of 1180 MPa or more, and a total elongation of 14% or more. Galvanized steel sheet.

5. In paragraph 4, The final microstructure of the above steel plate is characterized by having an area fraction of ferrite: 5 to 25%, retained austenite: 5 to 20%, a total of tempered martensite and bainite: 30 to 90%, and the remainder being at least one of pearlite and fresh martensite. Galvanized steel sheet.

6. In paragraph 1, The above steel sheet contains, in wt%, carbon (C): 0.2 to 0.4%, silicon (Si): more than 0% and 2.0% or less, manganese (Mn): 1.5 to 4.0%, aluminum (Al): more than 0 and 2.0% or less, phosphorus (P): more than 0 and 0.02% or less, sulfur (S): more than 0 and 0.005% or less, nitrogen (N): more than 0 and 0.006% or less, boron (B): more than 0 and 0.003% or less, and the remainder includes iron (Fe) and other unavoidable impurities. It is characterized by a yield strength of 1000 to 1350 MPa, a tensile strength of 1480 MPa or more, and a total elongation of 14% or more. Galvanized steel sheet.

7. In paragraph 6, The final microstructure of the above steel plate is characterized by having, in area fraction, ferrite: 0 to 5%, retained austenite: 5 to 35%, the sum of tempered martensite and bainite: 30 to 95%, and the remainder being at least one of pearlite and fresh martensite. Galvanized steel sheet.

8. In paragraph 1, The final microstructure of the above steel plate is characterized by satisfying the following equation 1: Galvanized steel sheet. (Formula 1) (X Pearlite + X Fresh Martensite ) / (100 - X Ferrite - X Tempered Martensite - X Bainite ) ≤ 0.30 (above X Pearlite is the value of the area fraction of pearlite (unit:%), and the above X Fresh Martensite is the area fraction value of fresh martensite (unit:%), and the above X Ferrite is the value of the area fraction of ferrite (unit:%), and X Tempered Martensite is the area fraction value of tempered martensite (unit:%), and the above X Bainite is the value of the area fraction of bainite (unit: %) A step of hot-rolling a steel material to provide a hot-rolled steel sheet, characterized in that the steel contains, by weight%, carbon (C): 0.1 to 0.4%, silicon (Si): more than 0% and 2.0% or less, manganese (Mn): 1.5 to 4.0%, aluminum (Al): more than 0 and 2.0% or less, phosphorus (P): more than 0 and 0.02% or less, sulfur (S): more than 0 and 0.005% or less, nitrogen (N): more than 0 and 0.006% or less, and boron (B): more than 0 and 0.003% or less, and the remainder includes iron (Fe) and other unavoidable impurities, but the sum of the content of silicon (Si) and the content of aluminum (Al) is 2.0% or less, and the ratio of the content of silicon (Si) and the content of aluminum (Al) is less than 4.0; A step of cold rolling the hot rolled steel sheet to provide a cold rolled steel sheet; A step of annealing the above cold rolled steel sheet; A step of cooling the above annealed heat-treated steel sheet at a first cooling rate; A step of rapidly cooling the above-described annealed steel plate to a temperature below the martensite transformation initiation temperature (Ms) at a second cooling rate greater than the first cooling rate; A reheating step of maintaining the rapidly cooled steel plate in a temperature range higher than the martensite transformation initiation temperature (Ms) and lower than the bainite transformation initiation temperature (Bs); A step of performing a plating process on the reheated steel plate to form a plating layer on the steel plate; and An alloying heat treatment step of maintaining the above-mentioned steel plate and the above-mentioned plating layer at a temperature range of 480 to 560°C and then cooling them to room temperature; Method for manufacturing galvanized steel sheet.

10. In paragraph 9, The above hot rolling is performed under the conditions of reheating temperature: 1150 to 1250℃, finishing rolling temperature: 800 to 950℃, and coiling temperature: 400 to 650℃. Method for manufacturing galvanized steel sheet.

11. In paragraph 9, The first cooling rate is lower than 20°C / s, and the second cooling rate is higher than 20°C / s. The slow cooling end temperature of the above slow cooling step is 650 to 750°C, and the rapid cooling end temperature of the above rapid cooling step is 200 to 300°C. Method for manufacturing galvanized steel sheet.

12. In paragraph 9, The above alloying heat treatment step includes a step of maintaining the temperature range of 480 to 560°C for 15 to 45 seconds and then cooling to room temperature at a third cooling rate, wherein the third cooling rate is greater than the first cooling rate and less than the second cooling rate. Method for manufacturing galvanized steel sheet.

13. In paragraph 9, The above steel contains, in wt%, carbon (C): 0.1 to 0.3%, silicon (Si): more than 0% and 2.0% or less, manganese (Mn): 1.5 to 3.0%, aluminum (Al): more than 0 and 2.0% or less, phosphorus (P): more than 0 and 0.02% or less, sulfur (S): more than 0 and 0.005% or less, nitrogen (N): more than 0 and 0.006% or less, boron (B): more than 0 and 0.003% or less, and the remainder includes iron (Fe) and other inevitable impurities. The step of annealing the cold rolled steel sheet includes a step of maintaining the cold rolled steel sheet in a temperature range of 800 to 850°C for 60 to 130 seconds. The final microstructure of the steel sheet implemented after performing the above alloying heat treatment step is, in terms of area fraction, ferrite: 30 to 50%, retained austenite: 5 to 20%, the sum of tempered martensite and bainite: 30 to 65%, and the remainder being at least one of pearlite and fresh martensite, and satisfying the following equation 1: The above-mentioned galvanized steel sheet is characterized by a yield strength of 600 to 760 MPa, a tensile strength of 980 MPa or more, and a total elongation of 21% or more. Method for manufacturing galvanized steel sheet. (Formula 1) (X Pearlite + X Fresh Martensite ) / (100 - X Ferrite - X Tempered Martensite - X Bainite ) ≤ 0.30 (above X Pearlite is the value of the area fraction of pearlite (unit:%), and the above X Fresh Martensite is the area fraction value of fresh martensite (unit:%), and the above X Ferrite is the value of the area fraction of ferrite (unit:%), and X Tempered Martensite is the area fraction value of tempered martensite (unit:%), and the above X Bainite is the value of the area fraction of bainite (unit: %) 14. In paragraph 9, The above steel contains, in wt%, carbon (C): 0.1 to 0.3%, silicon (Si): more than 0% and 2.0% or less, manganese (Mn): 2.0 to 4.0%, aluminum (Al): more than 0 and 2.0% or less, phosphorus (P): more than 0 and 0.02% or less, sulfur (S): more than 0 and 0.005% or less, nitrogen (N): more than 0 and 0.006% or less, boron (B): more than 0 and 0.003% or less, and the remainder includes iron (Fe) and other unavoidable impurities. The step of annealing the cold rolled steel sheet includes a step of maintaining the cold rolled steel sheet in a temperature range of 800 to 850°C for 60 to 130 seconds. The final microstructure of the steel sheet implemented after performing the above alloying heat treatment step is, in terms of area fraction, ferrite: 5 to 25%, retained austenite: 5 to 20%, the sum of tempered martensite and bainite: 30 to 90%, and the remainder being at least one of pearlite and fresh martensite, and satisfying the following equation 1: The above-mentioned galvanized steel sheet is characterized by a yield strength of 850 to 1070 MPa, a tensile strength of 1180 MPa or more, and a total elongation of 14% or more. Method for manufacturing galvanized steel sheet. (Formula 1) (X Pearlite + X Fresh Martensite ) / (100 - X Ferrite - X Tempered Martensite - X Bainite ) ≤ 0.30 (above X Pearlite is the value of the area fraction of pearlite (unit:%), and the above X Fresh Martensite is the area fraction value of fresh martensite (unit:%), and the above X Ferrite is the value of the area fraction of ferrite (unit:%), and X Tempered Martensite is the area fraction value of tempered martensite (unit:%), and the above X Bainite is the value of the area fraction of bainite (unit: %) 15. In paragraph 9, The above steel contains, in wt%, carbon (C): 0.2 to 0.4%, silicon (Si): more than 0 to 2.0%, manganese (Mn): 1.5 to 4.0%, aluminum (Al): more than 0 to 2.0%, phosphorus (P): more than 0 to 0.02% or less, sulfur (S): more than 0 to 0.005% or less, nitrogen (N): more than 0 to 0.006% or less, boron (B): more than 0 to 0.003% or less, and the remainder includes iron (Fe) and other unavoidable impurities. The step of annealing the cold rolled steel sheet includes a step of maintaining the cold rolled steel sheet in a temperature range of 850 to 900°C for 60 to 130 seconds. The final microstructure of the steel sheet implemented after performing the above alloying heat treatment step is, in terms of area fraction, ferrite: 0 to 5%, retained austenite: 5 to 35%, the sum of tempered martensite and bainite: 30 to 95%, and the remainder being at least one of pearlite and fresh martensite, and satisfying the following equation 1: The above-mentioned galvanized steel sheet is characterized by a yield strength of 1000 to 1350 MPa, a tensile strength of 1480 MPa or more, and a total elongation of 14% or more. Method for manufacturing galvanized steel sheet. (Formula 1) (X Pearlite + X Fresh Martensite ) / (100 - X Ferrite - X Tempered Martensite - X Bainite ) ≤ 0.30 (above X Pearlite is the value of the area fraction of pearlite (unit:%), and the above X Fresh Martensite is the area fraction value of fresh martensite (unit:%), and the above X Ferrite is the value of the area fraction of ferrite (unit:%), and X Tempered Martensite is the area fraction value of tempered martensite (unit:%), and the above X Bainite is the value of the area fraction of bainite (unit: %)

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